[compiler-rt] [msan] Variadic support for AArch64

Now with variadic support for msan on aarch6 there is no need for
XFAIL signal_stress_test anymore.  Also to garantee aligned stores
for the FP/SIMD arguments enforce the '__msan_va_arg_tls' alignment
to sizeof the SIMD register (16).

llvm-svn: 255496