profile: Rudimentary suppport for PGO instrumentation

This is fairly minimal support for instrumentation based PGO. The data
format is inefficient, and the output file name is hardcoded to
pgo-data.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@198638 91177308-0d34-0410-b5e6-96231b3b80d8