[gdb/tdep] Fix gdb.base/watchpoint-running.exp on {arm,ppc64le}-linux

When running test-case gdb.base/watchpoint-running on ppc64le-linux (and
similar on arm-linux), we get:
...
(gdb) watch global_var^M
warning: Error when detecting the debug register interface. \
  Debug registers will be unavailable.^M
Watchpoint 2: global_var^M
(gdb) FAIL: $exp: all-stop: hardware: watch global_var
FAIL: $exp: all-stop: hardware: watchpoint hit (timeout)
...

The problem is that ppc_linux_dreg_interface::detect fails to detect the
hardware watchpoint interface, because the calls to ptrace return with errno
set to ESRCH.

This is a feature of ptrace: if a call is done while the tracee is not
ptrace-stopped, it returns ESRCH.

Indeed, in the test-case "watch global_var" is executed while the inferior is
running, and that triggers the first call to ppc_linux_dreg_interface::detect.

And because the detection failure is cached, subsequent attempts at setting
hardware watchpoints will also fail, even if the tracee is ptrace-stopped.

The way to fix this is to make sure that ppc_linux_dreg_interface::detect is
called when we know that the thread is ptrace-stopped, which in the current
setup is best addressed by using target-specific post_attach and
post_startup_inferior overrides.  However, as we can see in
aarch64_linux_nat_target, that causes code duplication.

Fix this by:
- defining a new target hook low_init_process, called from
  linux_init_ptrace_procfs, which is called from both
  linux_nat_target::post_attach and linux_nat_target::post_startup_inferior,
- adding implementations for ppc_linux_nat_target and arm_linux_nat_target
  that detect the hardware watchpoint interface,
- replacing the aarch64_linux_nat_target implementations of post_attach and
  post_startup_inferior with a low_init_process implementation.

Tested on ppc64le-linux, arm-linux, aarch64-linux and x86_64-linux.

Co-Authored-By: Tom de Vries <tdevries@suse.de>
Approved-By: Luis Machado <luis.machado@arm.com>

PR tdep/31834
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31834
PR tdep/31705
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31705

(cherry picked from commit 50de502a)