MIPS: Use -mnan value for -mabs if not specified

MIPS: Use -mnan value for -mabs if not specified

On most hardware, FCSR.ABS2008 is set the value same with FCSR.NAN2008.
Let's use this behaivor by default in GCC, aka
	gcc -mnan=2008 -c fabs.c
will imply `-mabs=2008`.

And of course, `gcc -mnan=2008 -mabs=legacy` can continue workable
like previous.

gcc/ChangeLog

	* config/mips/mips.cc(mips_option_override): Set mips_abs to
	2008, if mips_abs is default and mips_nan is 2008.

gcc/testsuite/
	* gcc.target/mips/fabs-nan2008.c: New test.
	* gcc.target/mips/fabsf-nan2008.c: New test.