x86: Update VEC macros to complete API for evex/evex512 impls

x86: Update VEC macros to complete API for evex/evex512 impls

1) Copy so that backport will be easier.
2) Make section only define if there is not a previous definition
3) Add `VEC_lo` definition for proper reg-width but in the
   ymm/zmm0-15 range.
4) Add macros for accessing GPRs based on VEC_SIZE
        This is to make it easier to do think like:
        ```
            vpcmpb %VEC(0), %VEC(1), %k0
            kmov{d|q} %k0, %{eax|rax}
            test %{eax|rax}
        ```
        It adds macro s.t any GPR can get the proper width with:
            `V{upcase_GPR_name}`

        and any mask insn can get the proper width with:
            `{upcase_mask_insn_without_postfix}`

This commit does not change libc.so

Tested build on x86-64