RISC-V: Disable uint128_t testcase of SAT_MUL when rv32

RISC-V: Disable uint128_t testcase of SAT_MUL when rv32

The rv32 doesn't support __uint128, and then we will have
error like below during test.

error: '__int128' is not supported on this target.

Thus, we disable the uint128_t related test when rv32.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/sat/sat_arith.h: Add xlen check for
	uint128_t.
	* gcc.target/riscv/sat/sat_u_mul-run-1-u16-from-u128.c: Enable
	run test for rv64 only.
	* gcc.target/riscv/sat/sat_u_mul-run-1-u32-from-u128.c: Ditto.
	* gcc.target/riscv/sat/sat_u_mul-run-1-u64-from-u128.c: Ditto.
	* gcc.target/riscv/sat/sat_u_mul-run-1-u8-from-u128.c: Ditto.

Signed-off-by: Pan Li <pan2.li@intel.com>