arm: Fix sign of MVE predicate mve_pred16_t [PR 107674]

The ACLE defines mve_pred16_t as an unsigned short.  This patch makes sure GCC
treats the predicate as an unsigned type, rather than signed.

gcc/ChangeLog:

	PR target/107674
	* config/arm/arm-builtins.cc (arm_simd_builtin_type): Rewrite to use
	new qualifiers parameter and use unsigned short type for MVE predicate.
	(arm_init_builtin): Call arm_simd_builtin_type with qualifiers
	parameter.
	(arm_init_crypto_builtins): Likewise.

gcc/testsuite/ChangeLog:

	PR target/107674
	* gcc.target/arm/mve/mve_vpt.c: New test.