Fix Thumb-2 undef handling for mixed-arch kernels

In kernels which support v6 and v7 platforms in a single binary such as omap2plus_defconfig for example, undefined instruction exceptions taken on Thumb instructions are not processed correctly, because the kernel assumes at build-time that Thumb-2 won't be supported in userspace.

This series implements a workaround, by allowing the __und_usr handler to check the CPU architecture at runtime, in affected kernels.

Changes since RFC:
* Renamed the global variable to __cpu_architecture and provided cpu_architecture() as in inline function in <asm/system.h>. This brings back the old API, but more efficiently.
* Dropped all the patches required to handle the reverted API change.
* cpu_architecture is now declared __pure, which may lead to slightly more efficient code in some cases.
* Added BUG_ON() to cpu_architecture() so that if __cpu_architecture is not set early enough, it gets noticed.

Thanks to Tixy and Nicolas Pitre for their helpful feedback which helped slim down this series.

Dave Martin (3):
ARM: Make cpu_alignment into a global variable
ARM: entry: Remove unnecessary masking when decoding Thumb-2
instructions
ARM: entry: Fix Thumb-2 undef handling for multi-CPU kernels

 arch/arm/include/asm/system.h |   11 +++++++++-
 arch/arm/kernel/entry-armv.S  |   44 ++++++++++++++++++++++++++++++++++++----
 arch/arm/kernel/setup.c       |    5 +++-
 3 files changed, 53 insertions(+), 7 deletions(-)