MIPS/GAS: Use FCSR rather than RA with CFC1/CTC1

Fix an issue caused by commit f9419b05 ("MIPS gas: code cleanup"),
<https://sourceware.org/ml/binutils/2002-05/msg00192.html>, and replace
the incorrect use of RA with the CFC1 and CTC1 instructions with FCSR.
While the register referred by its number is $31 in both cases, these
instructions operate on the floating-point control register file rather
than general-purpose registers.

	gas/
	* config/tc-mips.c (FCSR): New macro.
	(macro) <M_TRUNCWS, M_TRUNCWD>: Use it in place of RA.