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

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