ld: Update function prototypes for compilers defaulting to -std=gnu23

Since GCC 15 defaults to -std=gnu23, update function prototypes in linker
tests for compilers defaulting to -std=gnu23.

	PR ld/32546
	* ld-shared/main.c (shlib_checkfunptr1): Update prototype for
	compilers defaulting to -std=gnu23.
	(shlib_checkfunptr2): Likewise.
	* ld-shared/sh1.c (shlib_checkfunptr1): Likewise.
	(shlib_checkfunptr2): Likewise.
	* ld-srec/sr1.c (fn1): Likewise.
	(fn2): Likewise.
	* ld-srec/sr2.c (fn1): Likewise.
	(fn2): Likewise.
	* ld-vsb/main.c (shlib_checkfunptr1): Likewise.
	(shlib_checkfunptr2): Likewise.
	* ld-vsb/sh1.c (hlib_checkfunptr1): Likewise.
	(shlib_checkfunptr2): Likewise.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>