Currently, the /proc/cpuinfo outputs the entire riscv,isa string which
is not ideal when we have multiple ISA extensions present in the ISA
string. Some of them may not be enabled in kernel as well.
Same goes for the single letter extensions as well which prints the
entire ISA string. Some of they may not be valid ISA extensions as
well (e.g 'su')

Parse only the valid & enabled ISA extension and print them.
Tested-by: NHeiko Stuebner <heiko@sntech.de>
Signed-off-by: NAtish Patra <atishp@rivosinc.com>
Reviewed-by: NAnup Patel <anup@brainfault.org>
Signed-off-by: NPalmer Dabbelt <palmer@rivosinc.com>