Set compile-time fallback hostname to "localhost"

This ensures that we have a universal unbranded fallback hostname.
The branded fallback hostname will be set in os-release(5) instead.

Reference: https://fedoraproject.org/wiki/Changes/FallbackHostname

systemd.spec

@@ -523,7 +523,7 @@ CONFIGURE_OPTS=(
         -Dversion-tag=%{version}-%{release}
         # https://bugzilla.redhat.com/show_bug.cgi?id=1906010
         -Dshared-lib-tag=%{version_no_tilde}-%{release}
-        -Dfallback-hostname=%[0%{?fedora}?"fedora":"localhost"]
+        -Dfallback-hostname="localhost"
         -Ddefault-dnssec=no
         -Ddefault-dns-over-tls=no
         # https://bugzilla.redhat.com/show_bug.cgi?id=1867830