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.

gcc/config/mips/mips.cc

@@ -20550,6 +20550,8 @@ mips_option_override (void)
   /* Set NaN and ABS defaults.  */
   if (mips_nan == MIPS_IEEE_754_DEFAULT && !ISA_HAS_IEEE_754_LEGACY)
     mips_nan = MIPS_IEEE_754_2008;
+  if (mips_abs == MIPS_IEEE_754_DEFAULT && mips_nan == MIPS_IEEE_754_2008)
+    mips_abs = MIPS_IEEE_754_2008;
   if (mips_abs == MIPS_IEEE_754_DEFAULT && !ISA_HAS_IEEE_754_LEGACY)
     mips_abs = MIPS_IEEE_754_2008;
 

gcc/testsuite/gcc.target/mips/fabs-nan2008.c

@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-mnan=2008" } */
+
+NOMIPS16 double
+fabs_2008 (double d)
+{
+  return __builtin_fabs (d);
+}
+
+/* { dg-final { scan-assembler "\tabs\\.d\t" } } */

gcc/testsuite/gcc.target/mips/fabsf-nan2008.c

@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-mnan=2008" } */
+
+NOMIPS16 float
+fabsf_2008 (float f)
+{
+  return __builtin_fabsf (f);
+}
+
+/* { dg-final { scan-assembler "\tabs\\.s\t" } } */