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/libfdt/Android.bp b/libs/libfdt/Android.bp
index b889ee5..5920d5d 100644
--- a/libs/libfdt/Android.bp
+++ b/libs/libfdt/Android.bp
@@ -37,6 +37,7 @@
         "libcore.rust_sysroot",
     ],
     rustlibs: [
+        "libcstr",
         "liblibfdt_bindgen",
         "libzerocopy_nostd",
     ],
@@ -61,6 +62,7 @@
     ],
     prefer_rlib: true,
     rustlibs: [
+        "libcstr",
         "liblibfdt",
     ],
 }
diff --git a/libs/libfdt/src/lib.rs b/libs/libfdt/src/lib.rs
index b369390..6cfe5f7 100644
--- a/libs/libfdt/src/lib.rs
+++ b/libs/libfdt/src/lib.rs
@@ -31,15 +31,9 @@
 use core::ops::Range;
 use core::ptr;
 use core::result;
+use cstr::cstr;
 use zerocopy::AsBytes as _;
 
-// TODO(b/308694211): Use cstr!() from vmbase
-macro_rules! cstr {
-    ($str:literal) => {{
-        core::ffi::CStr::from_bytes_with_nul(concat!($str, "\0").as_bytes()).unwrap()
-    }};
-}
-
 /// Error type corresponding to libfdt error codes.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum FdtError {
diff --git a/libs/libfdt/tests/api_test.rs b/libs/libfdt/tests/api_test.rs
index d76b1a4..63cbdee 100644
--- a/libs/libfdt/tests/api_test.rs
+++ b/libs/libfdt/tests/api_test.rs
@@ -17,23 +17,12 @@
 //! Integration tests of the library libfdt.
 
 use core::ffi::CStr;
+use cstr::cstr;
 use libfdt::{Fdt, FdtError, FdtNodeMut, Phandle};
 use std::ffi::CString;
 use std::fs;
 use std::ops::Range;
 
-// TODO(b/308694211): Use cstr!() from vmbase
-macro_rules! cstr {
-    ($str:literal) => {{
-        const S: &str = concat!($str, "\0");
-        const C: &::core::ffi::CStr = match ::core::ffi::CStr::from_bytes_with_nul(S.as_bytes()) {
-            Ok(v) => v,
-            Err(_) => panic!("string contains interior NUL"),
-        };
-        C
-    }};
-}
-
 const TEST_TREE_WITH_ONE_MEMORY_RANGE_PATH: &str = "data/test_tree_one_memory_range.dtb";
 const TEST_TREE_WITH_MULTIPLE_MEMORY_RANGES_PATH: &str =
     "data/test_tree_multiple_memory_ranges.dtb";