Fix PR19061, gdb hangs/spins-on-cpu when debugging any program on Alpha

Fix PR19061, gdb hangs/spins-on-cpu when debugging any program on Alpha

This fixes PR19061, where gdb hangs/spins-on-cpu when debugging any
program on Alpha.

(This patch is Uros' forward port of the patch from comment #5
of the PR [1].)

Patch was tested on alphaev68-linux-gnu, also tested with gcc's
testsuite, where it fixed all hangs in guality.exp and
simulate-thread.exp testcases.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=19061#c5

gdb/ChangeLog:
2017-12-15  Richard Henderson  <rth@redhat.com>
	    Uros Bizjak  <ubizjak@gmail.com>

	PR gdb/19061
	* alpha-tdep.c (alpha_software_single_step): Call
	alpha_deal_with_atomic_sequence here.
	(set_gdbarch_software_single_step): Set to
	alpha_software_single_step.
	* nat/linux-ptrace.h [__alpha__]: Define GDB_ARCH_IS_TRAP_BRKPT
	and GDB_ARCH_IS_TRAP_HWBKPT.