Add an environment variable to override the default profile output file.

Also rename the default output file from "pgo-data" to "default.profdata".
The ".profdata" suffix is consistent with the name of the new llvm-profdata
tool.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@201808 91177308-0d34-0410-b5e6-96231b3b80d8