[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

llvm/docs/ReleaseNotes.rst

@@ -123,6 +123,8 @@ Changes to the Hexagon Backend
 Changes to the LoongArch Backend
 --------------------------------
 
+* The `lp64s` ABI is supported now and has been tested on Rust bare-matal target.
+
 Changes to the MIPS Backend
 ---------------------------
 

llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp

@@ -1928,7 +1928,6 @@ static bool CC_LoongArch(const DataLayout &DL, LoongArchABI::ABI ABI,
   default:
     llvm_unreachable("Unexpected ABI");
   case LoongArchABI::ABI_ILP32S:
-  case LoongArchABI::ABI_LP64S:
   case LoongArchABI::ABI_ILP32F:
   case LoongArchABI::ABI_LP64F:
     report_fatal_error("Unimplemented ABI");
@@ -1937,6 +1936,8 @@ static bool CC_LoongArch(const DataLayout &DL, LoongArchABI::ABI ABI,
   case LoongArchABI::ABI_LP64D:
     UseGPRForFloat = !IsFixed;
     break;
+  case LoongArchABI::ABI_LP64S:
+    break;
   }
 
   // FPR32 and FPR64 alias each other.