gas: dw2gencfi: move some tc_* defines to the header file

Move the following three defines to the header file, so the SCFI
machinery can use them:
 - tc_cfi_frame_initial_instructions
 - tc_cfi_startproc
 - tc_cfi_endproc

Although, the symthesized CFI cannot be emitted when CFI_EMIT_target is
set, but in future, we may want to allow synthesized CFI to be emitted
to all outputs currently supported by GAS for DWARF CFI.

gas/
        * dw2gencfi.c: Move from ...
	* dw2gencfi.h: ... to here.