libgomp: Revise libgomp/Fortran support conditionals

Subversion r152931 (Git commit 0046b1a727f73f0848733e28219069426b2dc218)
"Fix libgomp without --enable-languages=fortran", and its follow-up
Subversion r156232 (Git commit d0709b6aeec6b00e71011502fa311751fc138147)
"configure.ac: Test for executability of GFORTRAN",
Subversion r156264 (Git commit 2122aa973ed2c829caee5b010a60ad01922a3650)
"configure.ac: Test for executability of _the first word_ of GFORTRAN"
fixed the libgomp target library build in case that
'--enable-languages=fortran' was not in effect: in that case, disable
libgomp/Fortran support.

These changes certainly do the right thing in GCC configurations where
'$GFORTRAN' would be expected to be "just compiled" (built with the GCC
top-level build system).  However, in case that it's "pre-installed" (provided
externally, as determined by the GCC top-level build system), these changes
didn't work, and instead kept libgomp/Fortran support disabled, instead of
enabling it.  "Pre-installed" 'GFORTRAN="x86_64-w64-mingw32-gfortran [...]"' is
not accepted by the current 'test -x [...]' check, for instance, as discussed
in "libgomp: Fix gfortran detection when program is found via PATH",
<https://inbox.sourceware.org/20260529021023.9764-1-peter0x44@disroot.org>
<https://forge.sourceware.org/gcc/gcc/pulls/172>,
<https://inbox.sourceware.org/bmm.hj9f44xa6a.gcc.gcc-TEST.Peter0x44.172.1.0@forge-stage.sourceware.org>.

That proposed "libgomp: Fix gfortran detection when program is found via PATH"
patch has the issue that it picks up the "gfortran [...] found via PATH" also
in the case when GCC is configured without '--enable-languages=fortran' in
effect, and it then still builds and installs the libgomp/Fortran support files
in GCC configurations without Fortran support enabled.

Instead of "libgomp: Fix gfortran detection when program is found via PATH" as
proposed, we may revise libgomp/Fortran support conditionals generally, and
handle that in a conceptually simpler way: revert the three aforementioned
commits (in other words: simply again assign 'FC="$GFORTRAN"' (like
'libgfortran/configure.ac' also is doing), and conditionalize 'USE_FORTRAN' on
whether '--enable-languages=fortran' is in effect.  In other words: in case
that '--enable-languages=fortran' is in effect, assume that the
'GFORTRAN=[...]' passed to the libgomp build is functional -- no matter where
it's coming from: "just compiled" or "pre-installed".

	PR libgomp/41418
	libgomp/
	* configure.ac (FC): Revert ealier changes; just set to '"$GFORTRAN"'.
	(USE_FORTRAN): Conditionalize on '--enable-languages=fortran' being in
	effect.
	* configure: Regenerate.

Reported-by: Peter Damianov <peter0x44@disroot.org>