Exception table infrastructure

Exception table infrastructure

To redirect control flow if a fault occurs.  Entries are registered with the
_ASM_EXTABLE() and placed in the .ex_table section, which is collected
together by the linker.

The .ex_table section is sorted on boot (to facilitate fast searching), and
searched in do_exception() when a fault or abort is encountered.  If a
matching entry is found, control flow is redirected and the exception returned
from.

Some of the changes are to make the two asm_macros.h files safe to include in
C code.  In addition, an extra selftest is added, making use of the exception
table infrastructure.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>