When QEMU testing is enabled, QEMU should be required

Since QEMU is needed to run tests with QEMU, if the program cannot
be found the build should stop and an error generated.

Author: dcandler

arm-runtimes/CMakeLists.txt

@@ -157,9 +157,9 @@ if(ENABLE_LIBC_TESTS OR ENABLE_COMPILER_RT_TESTS OR ENABLE_LIBCXX_TESTS)
 
     if(TEST_EXECUTOR STREQUAL qemu)
         if(TARGET_ARCH MATCHES "^aarch64")
-            find_program(QEMU_EXECUTABLE qemu-system-aarch64)
+            find_program(QEMU_EXECUTABLE qemu-system-aarch64 REQUIRED)
         else()
-            find_program(QEMU_EXECUTABLE qemu-system-arm)
+            find_program(QEMU_EXECUTABLE qemu-system-arm REQUIRED)
         endif()
 
         # Use colon as a separator because comma and semicolon are used for