configure: Avoid building TCG when not needed

configure: Avoid building TCG when not needed

Avoid building TCG when building only tools:

  ./configure --enable-tools --disable-system --disable-user

This saves us from running the soft-float tests enabled since
commit 76170102508.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200522172510.25784-3-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>