gdb/testsuite: Use _inferior_thread_count in gdb.threads/threadcrash.exp

gdb/testsuite: Use _inferior_thread_count in gdb.threads/threadcrash.exp

A linaro PR [1] reports that the gdb.threads/threadcrash.exp test-case fails
to cout the number of threads in the inferior:
...
FAIL: gdb.threads/threadcrash.exp: test_gcore: $thread_count == 7
FAIL: gdb.threads/threadcrash.exp: test_gcore: $thread_count == [llength $test_list]
...

Fix this by getting the convenience variable _inferior_thread_count as opposed
to calculating it based on the output of "info threads".

Tested on arm-linux and x86_64-linux.

Reviewed-By: Lancelot Six <lancelot.six@amd.com>
Approved-By: Tom de Vries <tdevries@suse.de>
[1] https://linaro.atlassian.net/browse/GNU-1120