[PowerPC] Make -pg generate calls to _mcount not mcount

At least on REL6 (Linux/glibc 2.12), the proper symbol for generating gprof
data is _mcount, not mcount. Prior to this change, compiling with -pg would
generate linking errors (because of unresolved references to mcount), after
this change -pg seems at least minimally functional.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205144 91177308-0d34-0410-b5e6-96231b3b80d8