ARC: MWDT: enable back BUILD_ASSERT macro

BUILD_ASSERT macro was disabled for MWDT toolchain from the
moment of adding MWDT support to Zephyr. Built-in _Static_assert
is now working fine for the most of the cases with MWDT toolchain
so we can use it in BUILD_ASSERT.

The only exception is _ARCH_MEM_PARTITION_ALIGN_CHECK macro
as it often used with variable addresses as parameters
which need to be checked at compile time. We disable
_ARCH_MEM_PARTITION_ALIGN_CHECK for MWDT toolchain so we can use
BUILD_ASSERT in other places.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>