[gdb] Fix segfault in for_each_block, part 1

[gdb] Fix segfault in for_each_block, part 1

When running test-case gdb.base/vfork-follow-parent.exp on powerpc64 (likewise
on s390x), I run into:
...
(gdb) PASS: gdb.base/vfork-follow-parent.exp: \
  exec_file=vfork-follow-parent-exit: target-non-stop=on: non-stop=off: \
  resolution_method=schedule-multiple: print unblock_parent = 1
continue^M
Continuing.^M
Reading symbols from vfork-follow-parent-exit...^M
^M
^M
Fatal signal: Segmentation fault^M
----- Backtrace -----^M
0x1027d3e7 gdb_internal_backtrace_1^M
        src/gdb/bt-utils.c:122^M
0x1027d54f _Z22gdb_internal_backtracev^M
        src/gdb/bt-utils.c:168^M
0x1057643f handle_fatal_signal^M
        src/gdb/event-top.c:889^M
0x10576677 handle_sigsegv^M
        src/gdb/event-top.c:962^M
0x3fffa7610477 ???^M
0x103f2144 for_each_block^M
        src/gdb/dcache.c:199^M
0x103f235b _Z17dcache_invalidateP13dcache_struct^M
        src/gdb/dcache.c:251^M
0x10bde8c7 _Z24target_dcache_invalidatev^M
        src/gdb/target-dcache.c:50^M
...
or similar.

The root cause for the segmentation fault is that linux_is_uclinux gives an
incorrect result: it should always return false, given that we're running on a
regular linux system, but instead it returns first true, then false.

In more detail, the segmentation fault happens as follows:
- a program space with an address space is created
- a second program space is about to be created. maybe_new_address_space
  is called, and because linux_is_uclinux returns true, maybe_new_address_space
  returns false, and no new address space is created
- a second program space with the same address space is created
- a program space is deleted. Because linux_is_uclinux now returns false,
  gdbarch_has_shared_address_space (current_inferior ()->arch ()) returns
  false, and the address space is deleted
- when gdb uses the address space of the remaining program space, we run into
  the segfault, because the address space is deleted.

Hardcoding linux_is_uclinux to false makes the test-case pass.

We leave addressing the root cause for the following commit in this series.

For now, prevent the segmentation fault by making the address space a refcounted
object.

This was already suggested here [1]:
...
A better solution might be to have the address spaces be reference counted
...

Tested on top of trunk on x86_64-linux and ppc64le-linux.
Tested on top of gdb-14-branch on ppc64-linux.

Co-Authored-By: Simon Marchi <simon.marchi@polymtl.ca>
PR gdb/30547
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30547

[1] https://sourceware.org/pipermail/gdb-patches/2023-October/202928.html