RISC-V: Use stdint-gcc.h in xtheadmemidx-helpers.h

stdint.h can be replaced with stdint-gcc.h to resolve some missing
system headers in non-multilib installations.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/xtheadmemidx-helpers.h:
	Replace stdint.h with stdint-gcc.h.