toolchain: add BUILD_ASSERT_MSG()

Like BUILD_ASSERT(), but with a message to emit on failure.

The base implementation swallows the message; compiler headers can
override it when they can do better.

Change-Id: Ib724e48554da77a51afa01468b1d5b7806f9de6b
Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>