PGO: allows the profile data file name to be specified by the LLVMPROF_OUTPUT

PGO: allows the profile data file name to be specified by the LLVMPROF_OUTPUT
environment variable.

This allows parallel make for profiling code, without it there are file
collisions as each parallel run uses the default file name.

There is already code in the runtime library to specify the output file name
via the command line, but this only works for programs which already process
argc/argv.  This patch builds on that support.

Patch by Alastair Murray.

llvm-svn: 167269