RISC-V: Tidy vendor core-v extension gas testcases

RISC-V: Tidy vendor core-v extension gas testcases

1. Combined testcases into one if they use same extention name.
2. Likewise for the fail testcases.
3. Renamed with x-cv prefix, just like what other vendors did.

gas/
	* testsuite/gas/riscv/cv-alu-*: Combined and renamed to
	x-cv-alu.  Likewise for fail testcases, to x-cv-alu-fail*.
	* testsuite/gas/riscv/cv-bi-*: Likewise, but renamed to
	x-cv-bi and x-cv-bi-fail.
	* testsuite/gas/riscv/cv-elw-*: Likewise, but renamed to
	x-cv-elw and x-cv-elw-fail.
	* testsuite/gas/riscv/cv-mac-*: Likewise, but renamed to
	x-cv-mac and x-cv-mac-fail.
	* testsuite/gas/riscv/cv-mem-*: Likewise, but renamed to
	x-cv-mem and x-cv-mem-fail.