crossgcc: Backport GNAT exception handler v1 patch

The GCC 10 GNAT toolchain uses a new exception handler ABI, so older
GNAT cannot be built with GCC 10. This patch backports the new
exception handler in libgnat to make GNAT able to be built.

The libgnat patch doesn't remove the old exception handler, so it can
still be built with older compilers.

The cross toolchain can now be built with GCC 10.1.0 in Arch Linux
(with the latest IASL in CB:38907 that can be built in Arch), and the
toolchain can build a working coreboot image with libgfxinit for HP
EliteBook 2560p.

The original and patched crossgcc built with Debian 10.4 GCC 8.3.0,
and the patched crossgcc built with Arch GCC 10.1.0 generate identical
coreboot images with `make BUILD_TIMELESS=1`.

Change-Id: I757158056bf4698d3c68715e026c226615bc70a1
Signed-off-by: Iru Cai <mytbk920423@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42158
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

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