xen/arm: Correctly support WARN_ON

xen/arm: Correctly support WARN_ON

Currently the hypervisor will hang if it hits a WARN_ON.

The implementation uses an undefined instruction, made ourself because ARM
don't provide one, to implement BUG/ASSERT/WARN_ON, and sets up the
different tables (one for each type) which contain useful information.

This is based on the x86 implementation (include/asm-x86/bug.h). Unfortunately
the structure can't be shared because many ARM{32,64} gcc versions doesn't
correctly support %c. The support for executing a function in an exception handler
is also keep unimplemented on ARM. Therefore, dump_execution_state is
implemented as WARN()

The current opcode used to go in exception mode may not be undefined on ARM64.
Use the instruction "brk" to generate a software debug exception.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>