Code like this:

  CORE_ADDR breaks[2] = {-1, -1};

... gives a warning with -Wnarrowing.  This patch changes all
instances of this to use CORE_ADDR_MAX instead.

gdb/ChangeLog
2018-08-27  Tom Tromey  <tom@tromey.com>

	* rs6000-tdep.c (ppc_deal_with_atomic_sequence): Use
	CORE_ADDR_MAX.
	* mips-tdep.c (mips_deal_with_atomic_sequence)
	(micromips_deal_with_atomic_sequence): Use CORE_ADDR_MAX.
	* arch/arm-get-next-pcs.c (thumb_deal_with_atomic_sequence_raw)
	(arm_deal_with_atomic_sequence_raw): Use CORE_ADDR_MAX.
	* alpha-tdep.c (alpha_deal_with_atomic_sequence): Use
	CORE_ADDR_MAX.
	* aarch64-tdep.c (aarch64_software_single_step): Use
	CORE_ADDR_MAX.