LoongArch: Add flogb.{s,d} instructions and expand logb{sf,df}2

LoongArch: Add flogb.{s,d} instructions and expand logb{sf,df}2

On LoongArch, flogb instructions extract the exponent of a non-negative
floating point value, but produces NaN for negative values.  So we need
to add a fabs instruction when we expand logb.

gcc/ChangeLog:

	* config/loongarch/loongarch.md (UNSPEC_FLOGB): New unspec.
	(type): Add flogb.
	(logb_non_negative<mode>2): New instruction template.
	(logb<mode>2): New define_expand.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/flogb.c: New test.