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/vmbase/Android.bp b/vmbase/Android.bp
index b2b1549..e682773 100644
--- a/vmbase/Android.bp
+++ b/vmbase/Android.bp
@@ -76,6 +76,7 @@
     rustlibs: [
         "libaarch64_paging",
         "libbuddy_system_allocator",
+        "libcstr",
         "libfdtpci",
         "libhyp",
         "liblibfdt",