[gdbsupport] Fix get_print_cell use in threads

PR gdb/33753 reports problems with get_print_cell when used from threads.

Fix this by making the two static variables in get_print_cell thread_local.

Tested on x86_64-linux.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33753