[ASan] Fix zero_page_pc test for PowerPC

Summary:
In the Power architecture, all branch instructions ignore the 2 least
significant bits of the target address. Consequently if you branch to an
invalid address, the address reported along with the SIGSEGV will have
been rounded down to a multiple of 4. Tweak this test accordingly.

This may fix the test for ARM too, in which case we could remove
the XFAIL, but I have no way of testing that.

Reviewers: kcc, willschm, glider, samsonov

Reviewed By: samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6167

llvm-svn: 221542