libs: Move cstr!() into its own crate

Make the macro host_supported, for unittests.

This overshadows the homonymous crate from crates.io [1] but importing
and depending on an external crate for <10 lines of code seems overkill.

[1]: https://crates.io/crates/cstr

Bug: 308694211
Test: atest libcstr.tests
Test: m libcstr
Test: m pvmfw_bin rialto_bin vmbase_example_bin
Change-Id: If1bf37034fc004a380b3ba528b9d76393a865d3e
diff --git a/libs/cstr/Android.bp b/libs/cstr/Android.bp
new file mode 100644
index 0000000..4ea87df
--- /dev/null
+++ b/libs/cstr/Android.bp
@@ -0,0 +1,36 @@
+package {
+    default_applicable_licenses: ["Android-Apache-2.0"],
+}
+
+rust_library_rlib {
+    name: "libcstr",
+    crate_name: "cstr",
+    defaults: ["avf_build_flags_rust"],
+    srcs: ["src/lib.rs"],
+    edition: "2021",
+    host_supported: true,
+    prefer_rlib: true,
+    target: {
+        android: {
+            no_stdlibs: true,
+            stdlibs: [
+                "libcompiler_builtins.rust_sysroot",
+                "libcore.rust_sysroot",
+            ],
+        },
+    },
+    apex_available: [
+        "//apex_available:platform",
+        "//apex_available:anyapex",
+    ],
+}
+
+rust_test {
+    name: "libcstr.tests",
+    crate_name: "libcstr_test",
+    defaults: ["avf_build_flags_rust"],
+    srcs: ["src/lib.rs"],
+    test_suites: ["general-tests"],
+    prefer_rlib: true,
+    rustlibs: ["libcstr"],
+}