Fixes PR85 for arm.

Fixes PR85 for arm.
* src/vm/jit/arm/linux/md-os.c (md_signal_handler_sigill): Check if
the trap instruction has been patched on the way to the signal
handler.
* src/vm/jit/arm/patcher.c (patcher_is_valid_trap_instruction_at): New
function.
* src/vm/jit/patcher-common.c (patcher_is_patched): Likewise.
(patcher_is_patched_at): Likewise.
* src/vm/jit/patcher-common.h: Likewise.