Get rid of out/common, make buildroot, qemu, gem5 in out/

Rationale: previously we had archs on toplevel, e.g. out/x86_64

However, host tools like QEMU and gem5 can reuse a lot of the common build
files across archs. Therefore, we save space and time by putting them into
a single directory.

Therefore, the toplevel out/x86_64 was inconsistent, better put arch
inside guest tools that need separate build trees instead, e.g.
out/buildroot/x86_64/

Also common was pretty obscure as a name to say the best.

build-all

@@ -14,8 +14,8 @@ while getopts A:G OPT; do
 done
 shift "$(($OPTIND - 1))"
 for arch in $archs; do
-  ./build --arch "$arch" --kernel-modules -l "$@"
   ./build-qemu --arch "$arch"
+  ./build --arch "$arch" --kernel-modules -l "$@"
   if "$gem5"; then
     ./build-gem5 --arch "$arch"
   fi