[llvm] [libc] add multi-platform pre-commit github actions (PR #119104)

[Tom Stellard via llvm-commits]

================
@@ -0,0 +1,76 @@
+# This workflow is for pre-commit testing of the LLVM-libc project.
+name: LLVM-libc Pre-commit Fullbuild Tests
+
----------------
tstellar wrote:

Can you add:
```
permissions:
  contents: read
```

To each new file you've added.  This is standard best practices for GitHub actions definitions. 

https://github.com/llvm/llvm-project/pull/119104