* som.c (som_prep_headers): If writing some form of an executable,

	* som.c (som_prep_headers): If writing some form of an executable,
	allocate and attach an exec header to the BFD private data.
	(som_begin_writing): Don't fill fields of the exec header based
	on BFD private data here (like the exec flags).  Do not write
	the exec header here.
	(som_write_headers): Instead do it here.
	* som.h (struct somdata): New field "exec_hdr" for the executable
	file header.
	(obj_som_exec_hdr): New accessor macro.