x86inc: Add REPX macro to repeat instructions/operations

When operating on large blocks of data it's common to repeatedly use
an instruction on multiple registers. Using the REPX macro makes it
easy to quickly write dense code to achieve this without having to
explicitly duplicate the same instruction over and over.

For example,

    REPX {paddw x, m4}, m0, m1, m2, m3
    REPX {mova [r0+16*x], m5}, 0, 1, 2, 3

will expand to

    paddw       m0, m4
    paddw       m1, m4
    paddw       m2, m4
    paddw       m3, m4
    mova [r0+16*0], m5
    mova [r0+16*1], m5
    mova [r0+16*2], m5
    mova [r0+16*3], m5