InstrProf: If libc is available, use it; no functionality change

It was misguided to plan to rely on __llvm_profile_write_buffer() in
__llvm_profile_write_file().  It's less complex to duplicate the writing
logic than to mmap the file.

Since it's here to stay, move `FILE*`-based writing logic into
InstrProfilingFile.c.

<rdar://problem/15943240>

llvm-svn: 204498