[gdb/build] Fix build breaker on mingw-w64

The mingw-w64 build breaks currently:
...
    In file included from gdb/cli/cli-cmds.c:58:
    gdbsupport/eintr.h: In function ‘pid_t gdb::waitpid(pid_t, int*, int)’:
    gdbsupport/eintr.h:77:35: error: ‘::waitpid’ has not been declared; \
                                     did you mean ‘gdb::waitpid’?
       77 |   return gdb::handle_eintr (-1, ::waitpid, pid, wstatus, options);
          |                                   ^~~~~~~
          |                                   gdb::waitpid
    gdbsupport/eintr.h:75:1: note: ‘gdb::waitpid’ declared here
       75 | waitpid (pid_t pid, int *wstatus, int options)
          | ^~~~~~~
...

This is a regression since commit 658a03e9e85 ("[gdbsupport] Add
gdb::{waitpid,read,write,close}"), which moved the use of ::waitpid from
run_under_shell, where it was used conditionally:
...
 #if defined(CANT_FORK) || \
       (!defined(HAVE_WORKING_VFORK) && !defined(HAVE_WORKING_FORK))
   ...
 #else
   ...
       int ret = gdb::handle_eintr (-1, ::waitpid, pid, &status, 0);
...
to gdb::waitpid, where it's used unconditionally:
...
inline pid_t
waitpid (pid_t pid, int *wstatus, int options)
{
  return gdb::handle_eintr (-1, ::waitpid, pid, wstatus, options);
}
...

Likewise for ::wait.

Guard these uses with HAVE_WAITPID and HAVE_WAIT.

Reproduced and tested by doing a mingw-w64 cross-build on x86_64-linux.

Reported-By: Simon Marchi <simark@simark.ca>
Co-Authored-By: Tom de Vries <tdevries@suse.de>