Bug 1632950 - Avoid failing the build when rustlib/etc is missing r=glandium

This was the case with my local rustc build today. We only use this directory for natvis files, it's fine to not have those.

Differential Revision: https://phabricator.services.mozilla.com/D72440

build/moz.configure/rust.configure

@@ -453,6 +453,7 @@ def rustc_natvis_ldflags(target, compiler_info, rustc):
         sysroot = check_cmd_output(rustc, '--print', 'sysroot').strip()
         etc = os.path.join(sysroot, 'lib/rustlib/etc')
         ldflags = []
+        if os.path.isdir(etc):
             for f in os.listdir(etc):
                 if f.endswith('.natvis'):
                     ldflags.append('-NATVIS:' + normsep(os.path.join(etc, f)))