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

Created by: NandniJamnadas

…igned.

* 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.