Fix gdb.Inferior.read_memory without execution (PR dap/30644)

Fix gdb.Inferior.read_memory without execution (PR dap/30644)

Andrew reported that the previous change to gdb.Inferior.read_memory &
friends introducing scoped_restore_current_inferior_for_memory broke
gdb.dap/stop-at-main.exp.  This is also reported as PR dap/30644.

The root of the problem is that all the methods that now use
scoped_restore_current_inferior_for_memory cause GDB to crash with a
failed assert if they are run on an inferior that is not yet started.

E.g.:

 (gdb) python i = gdb.selected_inferior ()
 (gdb) python i.read_memory (4,4)
 gdb/thread.c:626: internal-error: any_thread_of_inferior: Assertion `inf->pid != 0' failed.

This patch fixes the problem by removing
scoped_restore_current_inferior_for_memory's ctor ptid parameter and
the any_thread_of_inferior calls completely, and making
scoped_restore_current_inferior_for_memory switch inferior_ptid to a
pid ptid.

I was a little worried that some port might be assuming inferior_ptid
points at a thread in the xfer_partial memory access routines.  We
know that anything that supports forks must not assume that, due to
how detach_breakpoints works.  I looked at a number of xfer_partial
implementations, and didn't see anything that is looking at
inferior_ptid in a way that would misbehave.  I'm thinking that we
could go forward with this and just fix ports if they break.

While on some ports like on AMD GPU we have thread-specific address
spaces, and so when accessing memory for those address spaces, we must
have the right thread context (via inferior_ptid) selected, in
Inferior.read_memory, we only have the inferior to work with, so this
API as is can't be used to access thread-specific address spaces.
IOW, it can only be used to access the global address space that is
visible to both the CPU and the GPUs.

In proc-service.c:ps_xfer_memory, the other spot using
scoped_restore_current_inferior_for_memory, we're always accessing
per-inferior memory.

If we end up using scoped_restore_current_inferior_for_memory later to
set up the context to read memory from a specific thread, then we can
add an alternative ctor that takes a thread_info pointer, and make
inferior_ptid point to the thread, for example.

New test added to gdb.python/py-inferior.exp, exercising
Inferior.read_memory without execution.

No regressions on native and extended-gdbserver x86_64 GNU/Linux.

Reviewed-By: Tom Tromey <tom@tromey.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30644
Change-Id: I11309c5ddbbb51a4594cf63c21b3858bfd9aed19