Fix adjust_pc_after_break, remove still current thread check

Fix adjust_pc_after_break, remove still current thread check

On decr_pc_after_break targets, GDB adjusts the PC incorrectly if a
background single-step stops somewhere where PC-$decr_pc has a
breakpoint, and the thread that finishes the step is not the current
thread, like:

   ADDR1 nop <-- breakpoint here
   ADDR2 jmp PC

IOW, say thread A is stepping ADDR2's line in the background (an
infinite loop), and the user switches focus to thread B.  GDB's
adjust_pc_after_break logic confuses the single-step stop of thread A
for a hit of the breakpoint at ADDR1, and thus adjusts thread A's PC
to point at ADDR1 when it should not, and reports a breakpoint hit,
when thread A did not execute the instruction at ADDR1 at all.

The test added by this patch exercises exactly that.

I can't find any reason we'd need the "thread to be examined is still
the current thread" condition in adjust_pc_after_break, at least
nowadays; it might have made sense in the past.  Best just remove it,
and rely on currently_stepping().

Here's the test's log of a run with an unpatched GDB:

 35        while (1);
 (gdb) PASS: gdb.threads/step-bg-decr-pc-switch-thread.exp: next over nop
 next&
 (gdb) PASS: gdb.threads/step-bg-decr-pc-switch-thread.exp: next& over inf loop
 thread 1
 [Switching to thread 1 (Thread 0x7ffff7fc2740 (LWP 29027))](running)
 (gdb)
 PASS: gdb.threads/step-bg-decr-pc-switch-thread.exp: switch to main thread
 Breakpoint 2, thread_function (arg=0x0) at ...src/gdb/testsuite/gdb.threads/step-bg-decr-pc-switch-thread.c:34
 34        NOP; /* set breakpoint here */
 FAIL: gdb.threads/step-bg-decr-pc-switch-thread.exp: no output while stepping

gdb/ChangeLog:
2015-02-11  Pedro Alves  <pedro@codesourcery.com>

	* infrun.c (adjust_pc_after_break): Don't adjust the PC just
	because the event thread is not the current thread.

gdb/testsuite/ChangeLog:
2015-02-11  Pedro Alves  <pedro@codesourcery.com>

	* gdb.threads/step-bg-decr-pc-switch-thread.c: New file.
	* gdb.threads/step-bg-decr-pc-switch-thread.exp: New file.