Add bazel targets for libc/include/... tests.

These tests are compiled with `-DLIBC_FULL_BUILD`, since otherwise
inclusion of system headers can redefined macros defined by LLVM
libc.

Also, set `alwayslink=True` for `//libc/test:LibcUnitTest`. This ensures that the `main` function provided always gets linked into a test target. While not strictly necessary, it makes it so tests like https://github.com/llvm/llvm-project/blob/45d8759cbed0f216786729718608a8be72a505c6/libc/test/include/signbit_test.c will give a duplicate symbol error if they incorrectly depend on `LibcUnitTest`.

Author: jtstogel

utils/bazel/llvm-project-overlay/libc/test/UnitTest/BUILD.bazel

@@ -72,6 +72,9 @@ libc_test_library(
         "//libc:llvm_libc_macros_stdfix_macros",
         "//llvm:Support",
     ],
+    # Force linking in this library's `main()` to surface
+    # a duplicate symbol error if a test defines its own main.
+    alwayslink = True,
 )
 
 libc_test_library(