Add profile feedback build to git

Add profile feedback build to git

Add a gcc profile feedback build option "profile-all" to the main
Makefile. It simply runs the test suite to generate feedback data and the
recompiles the main executables with that. The basic structure is similar
to the existing gcov code.

gcc is often able to generate better code with profile feedback data. The
training load also doesn't need to be too similar to the actual load, it
still gives benefits.

The test suite run is unfortunately quite long. It would be good to find a
suitable subset that runs faster and still gives reasonable feedback.

For now the test suite runs single threaded (I had some trouble running
the test suite with -jX)

I tested it with git gc and git blame kernel/sched.c on a Linux kernel
tree. For gc I get about 2.7% improvement in wall clock time by using the
feedback build, for blame about 2.4%.  That's not gigantic, but not shabby
either for a very small patch.

If anyone has any favourite CPU intensive git benchmarks feel free to try
them too.

I hope distributors will switch to use a feedback build in their packages.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>