fix(windows): vendor audiopus_sys + patch libopus for clang-cl SIMD

cargo-xwin drives the Windows MSVC cross-compile via clang-cl, under
which CMake sets MSVC=1 — causing libopus 1.3.1's `if(NOT MSVC)` guards
to skip the per-file `-msse4.1` / `-mssse3` COMPILE_FLAGS that its x86
SIMD source files need. Clang-cl (unlike real cl.exe) still honors
Clang's target-feature system, so those files then fail to compile
with "always_inline function '_mm_cvtepi16_epi32' requires target
feature 'sse4.1'" errors across silk/NSQ_sse4_1.c, NSQ_del_dec_sse4_1.c,
and VQ_WMat_EC_sse4_1.c.

Earlier attempts to fix this downstream (cargo-xwin toolchain file,
override.cmake CMAKE_C_COMPILE_OBJECT <FLAGS> replace, CFLAGS env vars)
all failed because cargo-xwin rewrites override.cmake from scratch on
every `cargo xwin build` invocation and cmake-rs's -DCMAKE_C_FLAGS=
assembly happens before toolchain FORCE sets propagate.

Fixing it upstream at the source: vendor audiopus_sys 0.2.2 into
vendor/audiopus_sys, patch its bundled opus/CMakeLists.txt to introduce
an MSVC_CL var (true only when CMAKE_C_COMPILER_ID == "MSVC", i.e. real
cl.exe), and flip the eight `if(NOT MSVC)` SIMD guards to
`if(NOT MSVC_CL)`. Clang-cl then gets the GCC-style per-file flags and
the SSE4.1 sources build cleanly. Also flip the `if(MSVC)` global /arch
block at line 445 to `if(MSVC_CL)` so only cl.exe applies /arch:AVX and
clang-cl relies purely on per-file flags (no global/per-file mixing).

Wire via [patch.crates-io] in the workspace root Cargo.toml; the patch
is resolved relative to the workspace root as `vendor/audiopus_sys`.

Upstream context: xiph/opus#256, xiph/opus PR #257 (both stale).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/audiopus_sys/opus/m4/as-gcc-inline-assembly.m4

@@ -0,0 +1,98 @@
+dnl as-gcc-inline-assembly.m4 0.1.0
+
+dnl autostars m4 macro for detection of gcc inline assembly
+
+dnl David Schleef <ds@schleef.org>
+
+dnl AS_COMPILER_FLAG(ACTION-IF-ACCEPTED, [ACTION-IF-NOT-ACCEPTED])
+dnl Tries to compile with the given CFLAGS.
+dnl Runs ACTION-IF-ACCEPTED if the compiler can compile with the flags,
+dnl and ACTION-IF-NOT-ACCEPTED otherwise.
+
+AC_DEFUN([AS_GCC_INLINE_ASSEMBLY],
+[
+  AC_MSG_CHECKING([if compiler supports gcc-style inline assembly])
+
+  AC_TRY_COMPILE([], [
+#ifdef __GNUC_MINOR__
+#if (__GNUC__ * 1000 + __GNUC_MINOR__) < 3004
+#error GCC before 3.4 has critical bugs compiling inline assembly
+#endif
+#endif
+__asm__ (""::) ], [flag_ok=yes], [flag_ok=no])
+
+  if test "X$flag_ok" = Xyes ; then
+    $1
+    true
+  else
+    $2
+    true
+  fi
+  AC_MSG_RESULT([$flag_ok])
+])
+
+AC_DEFUN([AS_ASM_ARM_NEON],
+[
+  AC_MSG_CHECKING([if assembler supports NEON instructions on ARM])
+
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],[__asm__("vorr d0,d0,d0")])],
+                    [AC_MSG_RESULT([yes])
+                     $1],
+                    [AC_MSG_RESULT([no])
+                     $2])
+])
+
+AC_DEFUN([AS_ASM_ARM_NEON_FORCE],
+[
+  AC_MSG_CHECKING([if assembler supports NEON instructions on ARM])
+
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],[__asm__(".arch armv7-a\n.fpu neon\n.object_arch armv4t\nvorr d0,d0,d0")])],
+                    [AC_MSG_RESULT([yes])
+                     $1],
+                    [AC_MSG_RESULT([no])
+                     $2])
+])
+
+AC_DEFUN([AS_ASM_ARM_MEDIA],
+[
+  AC_MSG_CHECKING([if assembler supports ARMv6 media instructions on ARM])
+
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],[__asm__("shadd8 r3,r3,r3")])],
+                    [AC_MSG_RESULT([yes])
+                     $1],
+                    [AC_MSG_RESULT([no])
+                     $2])
+])
+
+AC_DEFUN([AS_ASM_ARM_MEDIA_FORCE],
+[
+  AC_MSG_CHECKING([if assembler supports ARMv6 media instructions on ARM])
+
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],[__asm__(".arch armv6\n.object_arch armv4t\nshadd8 r3,r3,r3")])],
+                    [AC_MSG_RESULT([yes])
+                     $1],
+                    [AC_MSG_RESULT([no])
+                     $2])
+])
+
+AC_DEFUN([AS_ASM_ARM_EDSP],
+[
+  AC_MSG_CHECKING([if assembler supports EDSP instructions on ARM])
+
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],[__asm__("qadd r3,r3,r3")])],
+                    [AC_MSG_RESULT([yes])
+                     $1],
+                    [AC_MSG_RESULT([no])
+                     $2])
+])
+
+AC_DEFUN([AS_ASM_ARM_EDSP_FORCE],
+[
+  AC_MSG_CHECKING([if assembler supports EDSP instructions on ARM])
+
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],[__asm__(".arch armv5te\n.object_arch armv4t\nqadd r3,r3,r3")])],
+                    [AC_MSG_RESULT([yes])
+                     $1],
+                    [AC_MSG_RESULT([no])
+                     $2])
+])