[gdb/testsuite] Fix tdlabel_re references

Commit 467a34bb ("gdb tests: Allow for "LWP" or "process" in thread IDs
from info threads") introduces a new global variable tdlabel_re, but fails to
indicate it's global when used in procs in four test-cases.

Fix this by adding "global tdlabel_re".

Tested on aarch64-linux.