vax: Emit prologue as rtl.

vax: Emit prologue as rtl.

Not that there's much rtl to emit, since the CALL instruction
interpreting the procedure entry mask does almost all the work.
However, it means we're no longer emitting dwarf2 via the
text-based entry points.

        * config/vax/vax.md (define_c_enum unspecv): New.  Define the
        VUNSPEC_* constants here instead of via define_constants.
        (VUNSPEC_PEM): New constant.
        (procedure_entry_mask): New insn.
        (prologue): New expander.
        * config/vax/vax.c (vax_add_reg_cfa_offset): New.
        (vax_expand_prologue): Rename from vax_output_function_prologue;
        emit rtl instead of text.
        (TARGET_ASM_FUNCTION_PROLOGUE): Remove.
        (print_operand): Add 'x' prefix.

From-SVN: r175869