Support up to 3 conditional branches in an atomic sequence

gdb currently supports 1 conditional branch inside a ppc larx/stcx
critical region. Unfortunately there is existing code that contains more
than 1, for example in the ppc64 Linux kernel:

c00000000003ac18 <.__hash_page_4K>:
...
c00000000003ac4c:       ldarx   r31,0,r6
c00000000003ac50:       andc.   r0,r4,r31
c00000000003ac54:       bne-    c00000000003aee8 <htab_wrong_access>
c00000000003ac58:       andi.   r0,r31,2048
c00000000003ac5c:       bne-    c00000000003ae0c <htab_bail_ok>
c00000000003ac60:       rlwinm  r30,r4,30,24,24
c00000000003ac64:       or      r30,r30,r31
c00000000003ac68:       ori     r30,r30,2304
c00000000003ac6c:       oris    r30,r30,4096
c00000000003ac70:       stdcx.  r30,0,r6

If we try to single step through this we get stuck forever because
the reservation is never set when we step over the stdcx.

The following patch bumps the number to 3 conditional branches + 1
terminating branch. With this patch applied I can single step through
the offending function in the ppc64 Linux kernel.

gdb/
2014-03-28  Anton Blanchard  <anton@samba.org>

	* breakpoint.h (MAX_SINGLE_STEP_BREAKPOINTS): New define.
	* rs6000-tdep.c (ppc_deal_with_atomic_sequence): Allow for more
	than two breakpoints.
	* breakpoint.c (insert_single_step_breakpoint): Likewise.
	(insert_single_step_breakpoint): Likewise.
	(single_step_breakpoints_inserted): Likewise.
	(cancel_single_step_breakpoints): Likewise.
	(detach_single_step_breakpoints): Likewise.
	(single_step_breakpoint_inserted_here_p): Likewise.
Add multiple branches to ppc64 single step through atomic sequence testcase

Test 3 conditional branches in an atomic sequence, 2 to the same
destination.

gdb/testsuite/
2014-03-28  Anton Blanchard  <anton@samba.org>

	* gdb.arch/ppc64-atomic-inst.s: Add second and third branch
	inside atomic sequence.
Add lbarx/stbcx., lharx/sthcx. and lqarx/stqcx. single stepping

Newer CPUs support byte, half word and quad word atomic update
sequences.

gdb/testsuite/
2014-03-28  Anton Blanchard  <anton@samba.org>

	* rs6000-tdep.c (ppc_deal_with_atomic_sequence): Add single step
	support for lbarx/stbcx., lharx/sthcx. and lqarx/stqcx.