LoongArch: Add fcopysign instructions

Add fcopysign.{s,d} with the names copysign{sf,df}3 so GCC will expand
__builtin_copysign{f,} to a single instruction.

Link: https://sourceware.org/pipermail/libc-alpha/2022-November/143177.html

gcc/ChangeLog:

	* config/loongarch/loongarch.md (UNSPEC_FCOPYSIGN): New unspec.
	(type): Add fcopysign.
	(copysign<mode>3): New instruction template.

gcc/testsuite/ChangeLog:

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