Make it possible to have different instrumented and feedback builds without copying gcda files around [pr93401]

Hi,
in GCC 8 we changed -fprofile-generate=<path> to use mangled absolute paths in
the <path> directory. This was necessary to avoid clashes of files when gcc is
executed from different directories to build different sources of same
filename.

However this made it difficult to build projects on setups where instrumented
build is done in one directory, feedback build in different and possibly
training happens in yet another directory structure.  This happens i.e. for
Firefox builds for month or two.

This patch adds -fprofile-prefix-path that can be used to inform gcc where the
root of build directory is and strip it form the gcda filenames.
This is similar to exisitng debug-prefix-map but without the map feature since
it seems useless for profile data.

We spent quite some time with Maritn Liska discussing options and found no
better solution.  I was looking how this work on LLVM and they produce single
profdata file which is then transformed into kind of simple database by
llvmprofdata tool.  This database keys functions by filename and symbol name.
If you arrane two files with same name define static variable with same symbol
name this gets messedup and result in wrong info. So I think this is not very
good solution and preffer the extra option.

Bootstrapped/regtested x86_64-linux. I plan to commit it later today if there
are no complains.

I suppose our manual could have some central section on profile feedback
explaining the whole setup at one place.

Honza

	PR gcov-profile/93401
	* common.opt (profile-prefix-path): New option.
	* coverae.c: Include diagnostics.h.
	(coverage_init): Strip profile prefix path.
	* doc/invoke.texi (-fprofile-prefix-path): Document.