headers: Use the right windows specific arm instruction for __debugbreak()

The udf arm instruction has a platform specific meaning; on Windows
udf #0xfe is mapped to an exception with STATUS_BREAKPOINT. (The
previous instruction form does stop execution in a debugger, but
continuing might not be possible, but with udf #0xfe it is possible
to continue after stopping.)

Signed-off-by: Martin Storsjö <martin@martin.st>