Given an invocation context block, a general-purpose register
    index such that 1 <= index < 16, and a quadword value gr_copy,
    writes the corresponding invocation context block general
    register and uses LIB$X86_PUT_INVO_REGISTERS to write to the
    actual context. The invocation context block remains unchanged
    if the routine fails. LIB$X86_SET_GR fails if
    LIB$X86_PUT_INVO_REGISTERS fails.

    Format
      LIB$X86_SET_GR (invo_context, index, gr_copy)