[ASan] Fix stack-overflow test for PowerPC

Summary:
Tweak the asan stack overflow heuristics to cope with PowerPC64 redzones,
which are larger than on x86-64: 288 bytes for big-endian and 512 bytes
for little-endian.

Reviewers: kcc, willschm, samsonov, eugenis

Reviewed By: samsonov, eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6168