[winasan] Fix strrchr interception with vs2019 CRT

strrchr in vs2019 CRT begins with unrecognized instructions.

VCRUNTIME140!strrchr:
4533c9         xor     r9d, r9d
4c8bc1         mov     r8, rcx

Patch by Christopher Reid!

Differential Revision: https://reviews.llvm.org/D60217

llvm-svn: 357725