common: rename --clang to --gem5-clang

The option was hard to understand, and this would cause ambiguity
if we ever support something else with the same option, e.g. rootfs
or userland executables.

README.adoc

@@ -12399,8 +12399,8 @@ TODO test properly, benchmark vs GCC.
 
 ....
 sudo apt-get install clang
-./build-gem5 --clang
-./run --clang --emulator gem5
+./build-gem5 --gem5-clang
+./run --emulator gem5 --gem5-clang
 ....
 
 ==== gem5 sanitation build