[msan] Don't hardcode 4kiB page size in msan_test.cc.

This breaks the valloc test on PowerPC, which has 64kiB pages.  Since
getting page size portably is nontrivial, and there's already a function
for that in __sanitizer, just use it.  Unfortunately, sanitizer_common.h
conflicts with the interface headers inclucded by msan_test.cc (and a few
of its own macros), so we have to declare it manually.

Differential Revision: http://reviews.llvm.org/D19227

llvm-svn: 266688