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)

1  –  Returned

    None.

2  –  Arguments

 invo_context

    OpenVMS usage: invo_context_blk
    type:          structure
    access:        modify
    mechanism:     by reference

    Address of a valid invocation context block.

 index

    OpenVMS usage: index
    type:          longword
    access:        read
    mechanism:     by value

    Index into the IREG array of the invocation context block.

 gr_copy

    OpenVMS usage: integer value
    type:          quadword
    access:        read
    mechanism:     by reference

    Address of a quadword that contains the value to be written to
    the invocation context block.