macros: Pass -Ccodegen-units=1 in RUSTFLAGS

This has good impact on performance and binary size at cost of
compilation time. As nice side effect, it fixes bug which breaks
compilation of binaries in Fedora Rawhide.

For example, ffsend binary went from 107M to 70M.

References: https://bugzilla.redhat.com/show_bug.cgi?id=1701339
Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>

data/macros.rust

@@ -2,7 +2,12 @@
 %__rustdoc %{_bindir}/rustdoc
 
 # Enable optimization, debuginfo, and link hardening.
-%__global_rustflags -Copt-level=3 -Cdebuginfo=2 -Clink-arg=-Wl,-z,relro,-z,now
+%__global_rustflags %{shrink:
+  -Copt-level=3
+  -Cdebuginfo=2
+  -Clink-arg=-Wl,-z,relro,-z,now
+  -Ccodegen-units=1
+}
 
 %__global_rustflags_toml [%{lua:
     for arg in string.gmatch(rpm.expand("%{__global_rustflags}"), "%S+") do