[Sanitizer] Fix segfaults during unwinding on SystemZ

Every now and then SystemZ programs built with ASan crash with

    ERROR: AddressSanitizer: stack-overflow on address 0x040000000000

for no apparent reason. The problem is that
BufferedStackTrace::UnwindFast() is specialized for SystemZ: it takes
register 14 from the frame, however, IsValidFrame() is not
specialized, and does not guarantee that frame[14] is going to be a
valid memory access.

Fix by introducing per-arch kFrameSize and using it in IsValidFrame().

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D85822