Added __builtin_riscv_cv_simd_neg_[h,b]

Issue [#54](https://github.com/openhwgroup/corev-gcc/issues/54) CORE-V:
SIMD __builtin_riscv_cv_simd_neg_h and __builtin_riscv_cv_simd_neg_b needs
implementing

Files Changed:

  * config/riscv/corev.def: Added new builtins.
  * config/riscv/corev.md: Likewise.
  * testsuite/gcc.target/riscv/cv-simd-neg-b-compile-1.c: Created.
  * testsuite/gcc.target/riscv/cv-simd-neg-h-compile-1.c: Likewise.

Signed-off-by: Mary Bennett <mary.bennett@embecosm.com>