Remove the sysroot restriction from building GCDAProfiling.c.

We really need the C standard library to be available to implement the
profiling runtime library reasonably, and replicating everything in the
SDKs tree really isn't addressing any problems we have. Notably, all of
the sanitizer runtimes take the same approach, and this isn't a library
which could end up in a bootstrapping problem where the system headers
aren't even available.

This will hopefully prevent subsequent changes which start using various
other bits of C standard library to make things more debuggable.

llvm-svn: 184798