CORE-V: SIMD Update: cv.avgu.sc{i}[.h,.b] builtins from signed to unsigned.

	* gcc/config/riscv/corev.def: Updated HI and QI to UHI and UQI.
	* gcc/config/riscv/corev.md: Updated predicate and constraint
	  from signed to unsigned.
	* gcc/doc/extend.texi: Updated documentation.
	* gcc/testsuite/gcc.target/riscv/cv-simd-avgu-sc-b-compile-1.c:
	  Updated tests.
	* gcc/testsuite/gcc.target/riscv/cv-simd-avgu-sc-h-compile-1.c:
	  Likewise.

Signed-off-by: Nandni Jamnadas <nandni.jamnadas@embecosm.com>