Improve performance of file I/O.

The fread / fwrite calls were happening for each timer. However, that could be
pretty expensive for a large number of timers. Instead, read and write the
timers in one call.

This gives ~10% speedup in compilation time.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@179990 91177308-0d34-0410-b5e6-96231b3b80d8