[flang] Enable LoongArch for x86CompatibleBehavior in floating point flag

Similar to D138503 for RISC-V which fix the flang-OldUnit test failure:

```
.../llvm-project/flang/unittests/Evaluate/real.cpp:504: FAIL:
FlagsToBits(prod.flags) == 0x18, not 0x10
0 0x800001 * 0xbf7ffffe
```

With this patch applied, `check-flang` all pass.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D143132