CORE-V: SIMD Update: Updated cv.pack and cv.pack.h builtins

Created by: NandniJamnadas

Updated cv.pack and cv.pack.h builtins from
__builtin_riscv_cv_simd_pack_h and __builtin_riscv_cv_simd_pack_h to
__builtin_riscv_cv_simd_packlo_h and __builtin_riscv_cv_simd_packhi_h
respectively.

* gcc/config/riscv/corev.def: Updated builtin and insn name.
* gcc/config/riscv/corev.md: Updated machine description insn name.
* gcc/doc/extend.texi: Updated documentation.
* gcc/testsuite/gcc.target/riscv/cv-simd-pack-compile-1.c: Updated tests.
* gcc/testsuite/gcc.target/riscv/cv-simd-pack-h-compile-1.c: Likewise
* gcc/testsuite/gcc.target/riscv/cv-xcvsimd-march-compile-1.c: Likewise.