assert: Make dead_code() work at link-time instead of compile-time

The dead_code() macro can be used to ensure that a certain code path is
compile-time eliminated (e.g. if you want to make sure it's never
executed for certain Kconfig combinations). Unfortunately, the current
implementation via __attribute__((error)) hits only at the GCC level.
This can catch code that can be compile-time eliminated based on state
within the same file, but it cannot be used in cases where a certain
library function is built but then garbage collected at link time.

This patch improves the macro by relying solely on the linker finding an
undefined reference. Unfortunately this makes the error message a little
less expressive (can no longer pass a custom string), but it is still
readable and one can add code comments next to the assertion to
elaborate further if necessary

Change-Id: I63399dc484e2150d8c027bc0256d9285e471f7cc
Signed-off-by: Julius Werner <jwerner@chromium.org>
---
M src/include/assert.h
M src/mainboard/google/gru/board.h
2 files changed, 17 insertions(+), 13 deletions(-)