msan: account for AVX state when unpoison ucontext_t

ucontext_t can be larger than its static size if it contains
AVX state and YMM/ZMM registers.
Currently a signal handler that tries to access that state
can produce false positives with random origins on stack.
Account for the additional ucontext_t state.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D116208