util/cacheinfo: Fix warning generated by clang

Clang generates the following warning on aarch64 host:

  CC      util/cacheinfo.o
/home/pranith/qemu/util/cacheinfo.c:121:48: warning: value size does not match register size specified by the constraint and modifier [-Wasm-operand-widths]
        asm volatile("mrs\t%0, ctr_el0" : "=r"(ctr));
                                               ^
/home/pranith/qemu/util/cacheinfo.c:121:28: note: use constraint modifier "w"
        asm volatile("mrs\t%0, ctr_el0" : "=r"(ctr));
                           ^~
                           %w0

Constraint modifier 'w' is not (yet?) accepted by gcc. Fix this by increasing the ctr size.

Tested-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Message-Id: <20170630153946.11997-1-bobby.prani@gmail.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>