gem5: build with ccache

CXX='ccache c++' does not work, CXX=/my/path/to/ccache/wrapper/c++
does not work, the only thing that works it to put it in your PATH.

I would rather put a wrapper that forwards to ccache in this repo to
avoid the external host dependency:

    #!/usr/bin/env bash
    ccache c++ "$@"

but then that script has to remove its directory from PATH or else
infinite recursion, and I have no patience to code that.

I considered using the ccache from Buildroot, but it does not setup the
nice /usr/lib PATH, so I gave up on that as well.

configure

@@ -40,6 +40,7 @@ wget \
 "
 if "$gem5"; then
   pkgs="$pkgs \
+ccache \
 gcc-aarch64-linux-gnu \
 gcc-arm-linux-gnueabi \
 libgoogle-perftools-dev \