toolchain: common: Allow multiple uses of BUILD_ASSERT() in same scope

BUILD_ASSERT() was always defining a type with the name
__build_assert_failure, causing issues if more than one assertion were
used in the same scope.

Also, use an enum instead of a typedef to avoid (possibly spurious)
warnings such as these:

  variably modified ‘__build_assert_failure1’ at file scope [-Werror]

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>