Auto merge of #21673 - gterzian:add_background_monitor, r=<try>

Add background hang monitor

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Early feedback welcome, still need to ~~actually integrate it in the constellation message loop, and~~ use it in Layout and Script...

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #16740 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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etc/ci/check_dynamic_symbols.py

@@ -15,7 +15,7 @@ import subprocess
 import sys
 
 symbol_regex = re.compile(b"D  \*UND\*\t(.*) (.*)$")
-allowed_symbols = frozenset([b'unshare', b'malloc_usable_size', b'__cxa_type_match'])
+allowed_symbols = frozenset([b'unshare', b'malloc_usable_size', b'__cxa_type_match', b'signal'])
 actual_symbols = set()
 
 objdump_output = subprocess.check_output([