tools: gmp: fix compilation with GCC15

tools: gmp: fix compilation with GCC15

Fedora 42 updated to GCC15 which now defaults to GNU23 as the default
instead of GNU17[1], and this breaks GMP compilation by failing to find
a working compiler test.

Its been fixed upstream [2][3], so backport the fix to fix GCC15 compilation.

[1] https://gcc.gnu.org/gcc-15/porting_to.html#c23
[2] https://gmplib.org/repo/gmp/rev/8e7bb4ae7a18
[3] https://gmplib.org/repo/gmp/rev/d66d66d82dbb

Link: https://github.com/openwrt/openwrt/pull/18506
Signed-off-by: Robert Marko <robimarko@gmail.com>