gdb/testsuite: Reduce gdb.threads/threadcrash.exp reliance on libc symbols

gdb/testsuite: Reduce gdb.threads/threadcrash.exp reliance on libc symbols

The test gdb.threads/threadcrash.exp demanded GDB to fully unwind and
print the names of all functions. However, some of the functions are
from the libc library, and so the test implicitly demanded libc symbols
to be available, and would fail otherwise, as was raised in PR
gdb/31293.

This commit changes it so we only explicitly check for functions that
are not provided by threadcrash.c if they are indeed available.

Tested on arm-linux and x86_64-linux.

Approved-By: Tom de Vries <tdevries@suse.de>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31293