[gdb/tdep] Fix arm thumb2 hw breakpoint

On an aarch64-linux system with 32-bit userland running in a chroot, and using
target board unix/mthumb I get:
...
(gdb) hbreak hbreak.c:27^M
Hardware assisted breakpoint 2 at 0x4004e2: file hbreak.c, line 27.^M
(gdb) PASS: gdb.base/hbreak.exp: hbreak
continue^M
Continuing.^M
Unexpected error setting breakpoint: Invalid argument.^M
(gdb) XFAIL: gdb.base/hbreak.exp: continue to break-at-exit after hbreak
...
due to this call in arm_linux_nat_target::low_prepare_to_resume:
...
          if (ptrace (PTRACE_SETHBPREGS, pid,
              (PTRACE_TYPE_ARG3) ((i << 1) + 1), &bpts[i].address) < 0)
            perror_with_name (_("Unexpected error setting breakpoint"));
...

This problem does not happen if instead we use a 4-byte aligned address.

This may or may not be a kernel bug.

Work around this by first using an inoffensive address bpts[i].address & ~0x7.

Likewise in arm_target::low_prepare_to_resume, which fixes the same fail on
target board native-gdbserver/mthumb.

While we're at it:
- use arm_hwbp_control_is_initialized in
  arm_linux_nat_target::low_prepare_to_resume,
- handle the !arm_hwbp_control_is_initialized case explicitly,
- add missing '_()' in arm_target::low_prepare_to_resume,
- make error messages identical between arm_target::low_prepare_to_resume and
  arm_linux_nat_target::low_prepare_to_resume,
- factor out sethbpregs_hwbp_address and sethbpregs_hwbp_control to
  make the implementation more readable.

Remove the tentative xfail added in d0af16d5 ("[gdb/testsuite] Add xfail in
gdb.base/hbreak.exp") by simply reverting the commit.

Tested on arm-linux.

Approved-By: Luis Machado <luis.machado@arm.com>
Tested-By: Luis Machado <luis.machado@arm.com>