[ASan] Improved stack overflow detection for PowerPC64

Summary:
AsanOnSIGSEGV has some heuristics for detecting stack overflow, but
they don't cope with a PowerPC store-with-update instruction which
modifies sp and stores to the modified address in one instruction.

This patch adds some PowerPC-specific code to check for this case.

This fixes the last few cases of the stack-overflow test.

Reviewers: kcc, samsonov, eugenis

Reviewed By: eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6253