util/cacheflush: Fix error generated by clang

util/cacheflush: Fix error generated by clang

When compiling qemu-fuzz-i386 on aarch64 host, clang reported the following
error:

../util/cacheflush.c:38:44: error: value size does not match register size
specified by the constraint and modifier [-Werror,-Wasm-operand-widths]
    asm volatile("mrs\t%0, ctr_el0" : "=r"(save_ctr_el0));
                                           ^
../util/cacheflush.c:38:24: note: use constraint modifier "w"
    asm volatile("mrs\t%0, ctr_el0" : "=r"(save_ctr_el0));
                       ^~
                       %w0

Modify the type of save_ctr_el0 to uint64_t to fix it.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Gan Qixin <ganqixin@huawei.com>
Message-Id: <20210115075656.717957-1-ganqixin@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>