[sanitizer][aarch64] Add cast to drop reliance on the type of uc_mcontext.__reserved

https://sourceware.org/bugzilla/show_bug.cgi?id=22742
uc_mcontext.__reserved probably should not be considered user visible API but
unfortunate it is: it is the only way to access cpu states of some Linux
asm/sigcontext.h extensions. That said, the declaration may be
long double __reserved[256]; (used by musl)
instead of
unsigned char __reserved[4096] __attribute__((__aligned__(16))); (glibc)
to avoid dependency on a GNU variable attribute.

compiler-rt/lib/sanitizer_common/sanitizer_linux.cpp

@@ -1773,7 +1773,7 @@ struct __sanitizer_esr_context {
 
 static bool Aarch64GetESR(ucontext_t *ucontext, u64 *esr) {
   static const u32 kEsrMagic = 0x45535201;
-  u8 *aux = ucontext->uc_mcontext.__reserved;
+  u8 *aux = reinterpret_cast<u8 *>(ucontext->uc_mcontext.__reserved);
   while (true) {
     _aarch64_ctx *ctx = (_aarch64_ctx *)aux;
     if (ctx->size == 0) break;