Use Clang rather than GCC for assembly. (#2377)

This avoids the hack for using aarch64-linux-gnu on Linux, and also
removes a dependency.

Also switched to using `cargo-objcopy`, as we require `cargo-binutils`
already and it makes the Makefiles simpler.

src/exercises/bare-metal/rtc/Makefile

@@ -12,14 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-UNAME := $(shell uname -s)
-ifeq ($(UNAME),Linux)
-	TARGET = aarch64-linux-gnu
-else
-	TARGET = aarch64-none-elf
-endif
-OBJCOPY = $(TARGET)-objcopy
-
 .PHONY: build qemu_minimal qemu qemu_logger
 
 all: rtc.bin
@@ -28,7 +20,7 @@ build:
 	cargo build
 
 rtc.bin: build
-	$(OBJCOPY) -O binary target/aarch64-unknown-none/debug/rtc $@
+	cargo objcopy -- -O binary $@
 
 qemu: rtc.bin
 	qemu-system-aarch64 -machine virt,gic-version=3 -cpu max -serial mon:stdio -display none -kernel $< -s

src/exercises/bare-metal/rtc/build.rs

@@ -16,10 +16,8 @@ use cc::Build;
 use std::env;
 
 fn main() {
-    #[cfg(target_os = "linux")]
-    env::set_var("CROSS_COMPILE", "aarch64-linux-gnu");
-    #[cfg(not(target_os = "linux"))]
     env::set_var("CROSS_COMPILE", "aarch64-none-elf");
+    env::set_var("CC", "clang");
 
     Build::new()
         .file("entry.S")