fix: use current_thread tokio runtime — avoid pthread_create SEGV on Android

Multi-thread tokio runtime crashes with SEGV_ACCERR in __init_tcb
during pthread_create on Android (static bionic stubs from CRT).
Switch to current_thread runtime which runs network I/O on the
calling thread without spawning additional OS threads.

Also: clean up build.rs — use only libc++_shared.so (dynamic),
remove getauxval_fix.c hack, remove static c++/c++abi linking.

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

.cargo/config.toml

@@ -1,9 +1,5 @@
 [target.aarch64-linux-android]
 linker = "aarch64-linux-android26-clang"
-rustflags = [
-    "-C", "target-feature=-outline-atomics",
-    "-C", "link-arg=-Wl,-z,lazy",
-]
 
 [target.armv7-linux-androideabi]
 linker = "armv7a-linux-androideabi26-clang"