Make CHECK_STATUS_PTHREAD signal-safe

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-06-26.12:40:49.115>
labels = ['interpreter-core', 'type-bug', '3.7']
title = 'Make CHECK_STATUS_PTHREAD signal-safe'
updated_at = <Date 2019-03-06.10:15:32.016>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2019-03-06.10:15:32.016>
actor = 'methane'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2017-06-26.12:40:49.115>
creator = 'pitrou'
dependencies = []
files = []
hgrepos = []
issue_num = 30766
keywords = []
message_count = 2.0
messages = ['296887', '337288']
nosy_count = 2.0
nosy_names = ['pitrou', 'methane']
pr_nums = ['2404']
priority = 'low'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30766'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7']