riscv: Add vendor extensions to /proc/cpuinfo

riscv: Add vendor extensions to /proc/cpuinfo

All of the supported vendor extensions that have been listed in
riscv_isa_vendor_ext_list can be exported through /proc/cpuinfo.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Evan Green <evan@rivosinc.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240719-support_vendor_extensions-v3-2-0af7587bbec0@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>