Move arm_apcs_32 extern declaration to header

Move arm_apcs_32 extern declaration to header

Instead of having several extern declarations for arm_apcs_32
in a few .c files, declare it in arm-tdep.h. This file is already
included from these .c files.

gdb/ChangeLog:

2019-09-19  Christian Biesinger  <cbiesinger@google.com>

	* arm-linux-nat.c: Remove extern declaration for arm_apcs_32.
	* arm-linux-tdep.c: Likewise.
	* arm-nbsd-nat.c: Likewise.
	* arm-tdep.h: Declare arm_apcs_32.