[libunwind] libunwind: Remove OS requirements from tests to make them run on more OSes (PR #167642)

[Alexander Richardson via cfe-commits]

================
@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// REQUIRES: linux && target={{riscv64-.+}}
+// REQUIRES: target-riscv64
----------------
arichardson wrote:

```suggestion
// REQUIRES: target={{riscv64-.+}}
```

https://github.com/llvm/llvm-project/pull/167642