Now hwcap.h and cpufeature.h are mutually including each other, and most of
the variable/API declarations in hwcap.h are implemented in cpufeature.c,
so, it's better to move them into cpufeature.h and leave only macros for
ISA extension logical IDs in hwcap.h.

BTW, the riscv_isa_extension_mask macro is not used now, so this patch
removes it.
Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Xiao Wang <xiao.w.wang@intel.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20231031064553.2319688-2-xiao.w.wang@intel.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>