smbios: move smbios code into a common folder

To share smbios among different architectures, this patch moves SMBIOS
code (smbios.c and smbios.h) from x86 specific folders into new
hw/smbios directories. As a result, CONFIG_SMBIOS=y is defined in
x86 default config files.

Acked-by: Gabriel Somlo <somlo@cmu.edu>
Tested-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Wei Huang <wei@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>