While investigating possible race conditions in the GCC testsuites
caused by bufferization issues, I wanted to investigate workarounds
similar to GDB's READ1 [1], and I noticed it was not always possible
to override EXPECT when running 'make check'.

This patch adds the missing support in various Makefiles.

I was not able to test the patch for all the libraries updated here,
but I confirmed it works as intended/needed for libstdc++.

libatomic, libitm, libgomp already work as intended because their
Makefiles do not have:
MAKEOVERRIDES=

Tested on (native) aarch64-linux-gnu, confirmed the patch introduces
the behaviour I want in gcc, g++, gfortran and libstdc++.

I updated (but could not test) libgm2, libphobos, libquadmath and
libssp for consistency since their Makefiles have MAKEOVERRIDES=

libffi, libgo, libsanitizer seem to need a similar update, but they
are imported from their respective upstream repo, so should not be
patched here.

[1] https://github.com/bminor/binutils-gdb/blob/master/gdb/testsuite/README#L269

2023-12-21  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* Makefile.in: Allow overriding EXEPCT.

	libgm2/
	* Makefile.am: Allow overriding EXEPCT.
	* Makefile.in: Regenerate.

	libphobos/
	* Makefile.am: Allow overriding EXEPCT.
	* Makefile.in: Regenerate.

	libquadmath/
	* Makefile.am: Allow overriding EXEPCT.
	* Makefile.in: Regenerate.

	libssp/
	* Makefile.am: Allow overriding EXEPCT.
	* Makefile.in: Regenerate.

	libstdc++-v3/
	* Makefile.am: Allow overriding EXEPCT.
	* Makefile.in: Regenerate.