riscv: hwcap: make ISA extension ids can be used in asm

So that ISA extensions can be used in assembly files, convert the
multi-letter RISC-V ISA extension IDs enums to macros.
In order to make them visible, move the #ifndef __ASSEMBLY__ guard
to a later point in the header

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230128172856.3814-4-jszhang@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>