Fix PR gdb/19676: Disable displaced stepping if /proc not mounted

Fix PR gdb/19676: Disable displaced stepping if /proc not mounted

On GNU/Linux archs that support displaced stepping, if /proc is not
mounted, GDB gets stuck not able to step past breakpoints:

 (gdb) c
 Continuing.
 dl_main (phdr=<optimized out>, phnum=<optimized out>, user_entry=<optimized out>, auxv=<optimized out>) at rtld.c:2163
 2163      LIBC_PROBE (init_complete, 2, LM_ID_BASE, r);
 Cannot find AT_ENTRY auxiliary vector entry.
 (gdb) c
 Continuing.
 dl_main (phdr=<optimized out>, phnum=<optimized out>, user_entry=<optimized out>, auxv=<optimized out>) at rtld.c:2163
 2163      LIBC_PROBE (init_complete, 2, LM_ID_BASE, r);
 Cannot find AT_ENTRY auxiliary vector entry.
 (gdb)

That's because GDB can't figure out where the scratch pad is.

This is a regression introduced by the earlier changes to make the
Linux native target always work in non-stop mode.

This commit makes GDB detect the case and fallback to stepping over
breakpoints in-line.

gdb/ChangeLog:
2016-03-15  Pedro Alves  <palves@redhat.com>

	PR gdb/19676
	* infrun.c (displaced_step_prepare): Also disable displaced
	stepping on NOT_SUPPORTED_ERROR.
	* linux-tdep.c (linux_displaced_step_location): If reading auxv
	fails, throw NOT_SUPPORTED_ERROR instead of generic error.