util/crossgcc/buildgcc: Fix GMP build on GCC 15

Currently building GMP with GCC 15 causes this error:
"configure: error: could not find a working compiler, see config.log
for details"

GCC 15 by default uses std=gnu23 but that causes the build of GMP to
fail. So hardcode it to always build GMP using std=gnu17.

Once GMP is fixed in their next release, we can remove this again.

source:
https://gmplib.org/list-archives/gmp-bugs/2024-November/005550.html

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: I7813ff18e8e486bece503652af69de1db93958b2
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87650
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/crossgcc/buildgcc

@@ -654,7 +654,9 @@ build_GMP() {
 	fi
 
 	# shellcheck disable=SC2086
-	CC="$(hostcc host)" CXX="$(hostcxx host)" \
+	# Bug: GMP currently does not support being build by GCC 15 (--std=gnu23)
+	#      Remove CFLAGS once it works again.
+	CC="$(hostcc host)" CXX="$(hostcxx host)" CFLAGS="-std=gnu17" \
 	../${GMP_DIR}/configure \
 		--disable-shared \
 		--enable-fat \