[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.