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.
Address a few of the issues in GCDAProfiling I noted when looking
through Bill's patch:

1) Correctly test the file descriptor after the sceond attempt at
   creating the file.
2) Make the filename a global so that we can issue error messages from
   other routines.
3) Check errno in several places and print it out so that errors are
   easier to track down.

I don't really expect any of these to fix the current failures I'm
seeing, but I'm hopeful they'll at least let me debug them.