[ASan] Fix zero_page_pc test for PowerPC

[foad]

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.

[samsonov]

LGTM

test/asan/TestCases/zero_page_pc.cc

 // Check that ASan correctly detects SEGV on the zero page.
 // RUN: %clangxx_asan %s -o %t && not %run %t 2>&1 | FileCheck %s
 
 // https://code.google.com/p/address-sanitizer/issues/detail?id=336
 // XFAIL: android
 // XFAIL: armv7l-unknown-linux-gnueabihf
 
 typedef void void_f();
 int main() {
-  void_f *func = (void_f *)0x7;
+  void_f *func = (void_f *)0x4;
   func();
-  // CHECK: {{AddressSanitizer: SEGV.*(pc.*0007)}}
+  // CHECK: {{AddressSanitizer: SEGV.*(pc.*0004)}}
   return 0;
 }