Fix sol-thread.c compilation on 32-bit Solaris

Fix sol-thread.c compilation on 32-bit Solaris

sol-thread.c fails to compile on 32-bit Solaris: there are several
instances of

In file included from /vol/src/gnu/gdb/hg/master/local/gdb/../gdbsupport/common-defs.h:203,
                 from /vol/src/gnu/gdb/hg/master/local/gdb/defs.h:28,
                 from /vol/src/gnu/gdb/hg/master/local/gdb/sol-thread.c:51:
/vol/src/gnu/gdb/hg/master/local/gdb/sol-thread.c: In member function ‘virtual void sol_thread_target::resume(ptid_t, int, gdb_signal)’:
/vol/src/gnu/gdb/hg/master/local/gdb/sol-thread.c:416:20: error: format ‘%ld’ expects argument of type ‘long int’, but argument 2 has type ‘ULONGEST’ {aka ‘long long unsigned int’} [-Werror=format=]
  416 |         warning (_("Specified thread %ld seems to have terminated"),
      |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/vol/src/gnu/gdb/hg/master/local/gdb/../gdbsupport/gdb_locale.h:28:29:
note: in definition of macro ‘_’
   28 | # define _(String) gettext (String)
      |                             ^~~~~~
/vol/src/gnu/gdb/hg/master/local/gdb/sol-thread.c:416:40: note: format
string is defined here
  416 |         warning (_("Specified thread %ld seems to have terminated"),
      |                                      ~~^
      |                                        |
      |                                        long int
      |                                      %lld

Fixed by using pulongest () instead.

Tested on i386-pc-solaris2.11, amd64-pc-solaris2.11,
sparc-sun-solaris2.11, and sparcv9-sun-solaris2.11 (together with
Simon's patch for PR build/29798).