[asan] Fix dynamic allocas unpoisoning on PowerPC64.

For PowerPC64 we cannot just pass SP extracted from @llvm.stackrestore to
_asan_allocas_unpoison due to specific ABI requirements
(http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.html#DYNAM-STACK).
This patch adds the value returned by @llvm.get.dynamic.area.offset to
extracted from @llvm.stackrestore stack pointer, so dynamic allocas unpoisoning
stuff would work correctly on PowerPC64.

Patch by Max Ostapenko.

Differential Revision: http://reviews.llvm.org/D15108

llvm-svn: 254707