LoongArch: Replace UNSPEC_FCOPYSIGN with copysign RTL

LoongArch: Replace UNSPEC_FCOPYSIGN with copysign RTL

When I added copysign support for LoongArch (r13-3702), we did not have
a copysign RTL insn, so I had to use UNSPEC to represent the copysign
instruction. Now the copysign RTX code has been added in r14-1586, so
this patch removes those UNSPECs, and it uses the native RTL copysign
insn.

Inspired by rs6000 patch "Cleanup: Replace UNSPEC_COPYSIGN with copysign
RTL" [1] from Michael Meissner.

[1]: https://gcc.gnu.org/pipermail/gcc-patches/2023-September/631701.html

gcc/ChangeLog:

	* config/loongarch/loongarch.md (UNSPEC_FCOPYSIGN): Delete.
	(copysign<mode>3): Use copysign RTL instead of UNSPEC.