RISC-V: Documnet the list of supported extensions

RISC-V: Documnet the list of supported extensions

Try to list all supported extensions: name, version and few description
for each extension.

v2 changes:
 - Fix several typo.
 - Add expantion info for vector crypto extensions.
 - Drop zvl8192b, zvl16384b, zvl32768b and zvl65536b.
 - Aadd zicntr and zihpm

gcc/ChangeLog:

	* doc/invoke.texi (RISC-V Options): Add list of supported
	extensions.