[ Upstream commit 04f264d3 ]

We have a need to override the definition of
barrier_before_unreachable() for MIPS, which means we either need to add
architecture-specific code into linux/compiler-gcc.h or we need to allow
the architecture to provide a header that can define the macro before
the generic definition. The latter seems like the better approach.

A straightforward approach to the per-arch header is to make use of
asm-generic to provide a default empty header & adjust architectures
which don't need anything specific to make use of that by adding the
header to generic-y. Unfortunately this doesn't work so well due to
commit 28128c61 ("kconfig.h: Include compiler types to avoid missed
struct attributes") which caused linux/compiler_types.h to be included
in the compilation of every C file via the -include linux/kconfig.h flag
in c_flags.

Because the -include flag is present for all C files we compile, we need
the ar...