MIPS/GAS: Use frag symbol/offset directly in fixup creation

MIPS/GAS: Use frag symbol/offset directly in fixup creation

There is no need to use a helper expression in the creation of fixups
made from a frag's symbol and offset, because a simple `symbol+offset'
expression can be handled directly, with the use of a `fix_new' rather
than a `fix_new_exp' call.  Rewrite `md_convert_frag' using `fix_new'
then and remove all the unneeded helper expressions, simplifying code.

	gas/
	* config/tc-mips.c (md_convert_frag): Rewrite `fix_new_exp'
	calls in terms of `fix_new'.