[rust] Disable neon in aarch64 builds

Ensure that the generated aarch64 rust code does not make use of
floating point registers, pass this option in vai RUSTCFLAGS variable.
This will need more general support to add other architectures.

Signed-off-by: David Brown <david.brown@linaro.org>

Author: d3zd3z

lib/rust_support/rules.mk

@@ -85,6 +85,7 @@ $(CARGO_CONFIG): $(CARGO_CONFIG).phony
 		echo "# AUTOGENERATED -- edit $(MODULE_SRCDIR)/rules.mk instead"; \
 		echo "[build]"; \
 		echo 'target = "$(RUST_TARGET_PATH)"'; \
+		echo 'rustflags = [ "-Ctarget-feature=-neon"]'; \
 		echo; \
 		echo "[env]"; \
 		echo 'GLOBAL_INCLUDES = { value = "$(call TOML_ESC,$(GLOBAL_INCLUDES))", force = true }'; \