[gdb] Block SIGTERM in worker threads

With gdb build with gcc-12 and -fsanitize=thread, and test-case
gdb.base/gdb-sigterm.exp, I run into:
...
WARNING: ThreadSanitizer: data race (pid=9722)^M
  Write of size 4 at 0x00000325bc68 by thread T1:^M
  #0 handle_sigterm(int) src/gdb/event-top.c:1211 (gdb+0x8ec01f)^M
  ...
  Previous read of size 4 at 0x00000325bc68 by main thread:^M
    [failed to restore the stack]^M
^M
  Location is global 'sync_quit_force_run' of size 4 at \
  0x00000325bc68 (gdb+0x325bc68)^M
  ...
SUMMARY: ThreadSanitizer: data race gdb/event-top.c:1211 in \
  handle_sigterm(int)^M
...
and 3 more data races involving handle_sigterm and locations:
- active_ext_lang
- quit_flag
- heap block of size 40
  (XNEW (async_signal_handler) in create_async_signal_handler)

This was reported in PR29297.

The testcase executes a "kill -TERM $gdb_pid", which generates a
process-directed signal.

A process-directed signal can be delivered to any thread, and what we see
here is the fallout of the signal being delivered to a worker thread rather
than the main thread.

Fix this by blocking SIGTERM in the worker threads.

[ I have not been able to reproduce this after it occurred for the first time,
so unfortunately I cannot confirm that the patch fixes the problem. ]

Tested on x86_64-linux, with and without -fsanitize=thread.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29297

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