[LoongArch] Move lp64s out of the unimplemented calling conv list

lp64s is same as lp64d execpt that floating point arguments and return
values are always passed via GPRs or stack which means `UseGPRForFloat`
is always `true` in `CC_LoongArch` for lp64s.

One motivation of this change is to build linux which uses
`-msoft-float` and `-mabi=lp64s` [1].

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/loongarch/Makefile?h=v6.4-rc1#n49

Reviewed By: xen0n, hev

Differential Revision: https://reviews.llvm.org/D150417