[sanitizer][test] Fix FastUnwindTest on LoongArch

[tangyouling]

Summary

Fixes the FastUnwindTest unit test for LoongArch.
This change is similar to RISCV D90574.

The following test cases pass after applying the patch:

$ ./runtimes/runtimes-bins/compiler-rt/lib/sanitizer_common/tests/Sanitizer-loongarch64-Test
...
[  FAILED  ] FastUnwindTest.Basic
[  FAILED  ] FastUnwindTest.FramePointerLoop
[  FAILED  ] FastUnwindTest.MisalignedFramePointer
[  FAILED  ] FastUnwindTest.FPBelowPrevFP
[  FAILED  ] FastUnwindTest.CloseToZeroFrame

[xen0n]

Hmm, better yet, place LoongArch define ahead of the RISCV define, so the list appears alphabetically sorted.

[tangyouling]

Address @xen0n's comments.

compiler-rt/lib/sanitizer_common/tests/sanitizer_stacktrace_test.cpp

protected:
   const uptr fake_stack_size = 10;
   uhwptr start_pc;
 
   uhwptr fake_bp;
   uhwptr fake_top;
   uhwptr fake_bottom;
   BufferedStackTrace trace;
 
-#if defined(__riscv)
+#if defined(__riscv) || defined(__loongarch__)
   const uptr kFpOffset = 4;
   const uptr kBpOffset = 2;
 #else
   const uptr kFpOffset = 2;
   const uptr kBpOffset = 0;
 #endif
 
  private: