This also puts kMachExceptionCodes into mach_extensions.h, because a
form of MACH_EXCEPTION_CODES that’s the right type
(exception_behavior_t) has now shown its use in more than one file.
TEST=util_test ExcClientVariants.UniversalExceptionRaise
R=rsesek@chromium.org
Review URL: https://codereview.chromium.org/567283002