MIPS16/GAS: Factor out duplicate symbol value conversion code

MIPS16/GAS: Factor out duplicate symbol value conversion code

Factor out and consolidate duplicate section-relative to PC-relative
symbol value conversion in `mips16_extended_frag' and `md_convert_frag'
used for MIPS16 relaxation, observing that the final calculation in the
latter function implies `stretch == 0'.  Sanitize the formatting of code
moved.

	gas/
	* config/tc-mips.c (mips16_pcrel_val): New function, factored
	out from...
	(mips16_extended_frag): ... here.
	(md_convert_frag): Use `mips16_pcrel_val' rather than repeated
	code in MIPS16 relaxation, with `stretch' hardcoded to 0.