src/arch/x86: Build mainboard acpi_tables source if present

src/arch/x86: Build mainboard acpi_tables source if present

Current build rules require adding blank acpi_tables in some of the
mainboards (eg. octopus, hatch). Update the build rules to compile the
acpi_tables.c only if it is present. This will help to avoid adding
blank acpi_tables.c source file.

BUG=None
TEST=Build test with octopus and hatch without blank acpi_table.c file.

Change-Id: I7dfacc6f4c737699b22acd96e17c9426d33574bd
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37774
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Justin TerAvest <teravest@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Aamir Bohra <aamir.bohra@intel.com>