InstrProf: support PowerPC64

Summary:
I don't know anything about profiling but it seems to work out of the
box on PowerPC64. At least "make check-profile" works.

A few tests needed tweaking because PowerPC64 IR declares main with
"define signext i32 @main" instead of just "define i32 @main".

This also fixes the asan asan_and_llvm_coverage_test test, which
compiles with -coverage so requires that a profiling version of
libclang_rt has been built.

Reviewers: dexonsmith, kcc, samsonov

Reviewed By: samsonov

Subscribers: samsonov, llvm-commits

Differential Revision: http://reviews.llvm.org/D6233