aarch64: Update the mangling of single SVE vectors and predicates

aarch64: Update the mangling of single SVE vectors and predicates

GCC was implementing an old mangling scheme for single SVE
vectors and predicates (based on the Advanced SIMD one).
The final definition instead put them in the vendor built-in
namespace via the "u" prefix.

gcc/
	* config/aarch64/aarch64-sve-builtins.cc (DEF_SVE_TYPE): Add a
	leading "u" to each mangled name.

gcc/testsuite/
	* g++.target/aarch64/sve/acle/general-c++/mangle_1.C: Add a leading
	"u" to the mangling of each SVE vector and predicate type.
	* g++.target/aarch64/sve/acle/general-c++/mangle_2.C: Likewise.
	* g++.target/aarch64/sve/acle/general-c++/mangle_3.C: Likewise.
	* g++.target/aarch64/sve/acle/general-c++/mangle_5.C: Likewise.