[Sanitizer] Disable -Wframe-larger-than on SystemZ

SystemZ builds show -Wframe-larger-than warnings in two functions:
 'sanitizer::SuspendedThreadsListLinux::GetRegistersAndSP'
 'sanitizer::SizeClassAllocator32<__sanitizer::AP32>::PopulateFreeList'

In both cases, the frame size looks correct; each of the functions has
a large local variable that brings the frame size close to the limit
even on x86, and the extra 160 bytes of the default register save areas
on SystemZ pushes it over the limit.

PowerPC and MIPS already disable this warning; do the same on SystemZ.

Differential Revision: https://reviews.llvm.org/D66021

llvm-svn: 369543