qemuv8: fix to enable Rust by default on x86_64 hosts only

Change $(UNAME_M) to $(shell uname -m) to detect host, so that
"RUST_ENABLE" can be correctly set on x86_64 hosts.

Signed-off-by: Bing Zhu <bing.zhu@intel.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Tested-by: Jerome Forissier <jerome.forissier@linaro.org>

qemu_v8.mk

@@ -31,7 +31,7 @@ endif
 
 # Option to enable Rust examples
 # Currently supported only on x86_64 hosts
-ifeq ($(UNAME_M),x86_64)
+ifeq ($(shell uname -m),x86_64)
 RUST_ENABLE ?= y
 endif