rust: use optimization level s when building for size

In Nick's experience:

> z has created larger images than s due to LLVM aggressively
> emitting libcalls, regardless of the cost of call setup.
> See also commit a75bb4e ("Revert "kbuild: use -Oz
> instead of -Os when using clang").

This also aligns it to the C side.

Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

Makefile

@@ -809,7 +809,7 @@ KBUILD_CFLAGS += -O3
 KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := 3
 else ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
 KBUILD_CFLAGS += -Os
-KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := z
+KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := s
 endif
 
 # Always set `debug-assertions` and `overflow-checks` because their default