rust: pass correct target to bindgen on Usermode Linux

Usermode Linux uses "um" as primary architecture name and the underlying
physical architecture is provided in "SUBARCH".

Resolve the target architecture flags through that underlying
architecture.

This is the same pattern as used by scripts/Makefile.clang from which
the bindgen flags are derived.

[ David says:

      (...) this is enough to get Rust-for-Linux working with gcc under
      64-bit UML on my system.

    - Miguel ]

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: David Gow <davidgow@googl.ecom>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Link: https://lore.kernel.org/r/20250208-rust-kunit-v1-1-94a026be6d72@weissschuh.net
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: t-8ch

rust/Makefile

@@ -274,6 +274,7 @@ bindgen_skip_c_flags := -mno-fp-ret-in-387 -mpreferred-stack-boundary=% \
 # Derived from `scripts/Makefile.clang`.
 BINDGEN_TARGET_x86	:= x86_64-linux-gnu
 BINDGEN_TARGET_arm64	:= aarch64-linux-gnu
+BINDGEN_TARGET_um	:= $(BINDGEN_TARGET_$(SUBARCH))
 BINDGEN_TARGET		:= $(BINDGEN_TARGET_$(SRCARCH))
 
 # All warnings are inhibited since GCC builds are very experimental,