Generate an error if machine/armreg.h is included without sys/cdefs.h

machine/armreg.h requires access to the ARM_ARCH macro, which is not
always properly defined (especially by gcc 4.2.1). We should include
sys/cdefs.h in order to get the definitions in machine/acle-compat.h,
which would properly define the ARM_ARCH macro in these cases.

So, in cases where machine/armreg.h is included without _SYS_CDEFS_H_
being defined - generate an #error.

Reviewed by: andrew
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D8460