Add header file for XCVbitmanip

Files Changed:

• gcc/config.gcc: Add header file.
• gcc/config/riscv/riscv_corev_bitmanip.h: Likewise.
• gcc/testsuite/gcc.target/riscv/bitmanip-c-api.c: New Test.
• config/riscv/corev.md: Update RTL for cv.ff1 and cv.clb.