[compiler-rt] Fix FORTIFY_SOURCE -> _FORTIFY_SOURCE reference (NFC)

As pointed out by maskray.

Fixes: 8ab7625
(cherry picked from commit ca50897)

Author: thesamesam

compiler-rt/test/lit.common.cfg.py

@@ -738,5 +738,5 @@ def is_windows_lto_supported():
 # possibly be present in system and user configuration files, so disable
 # default configs for the test runs. In particular, anything hardening
 # related is likely to cause issues with sanitizer tests, because it may
-# preempt something we're looking to trap (e.g. FORTIFY_SOURCE vs our ASAN).
+# preempt something we're looking to trap (e.g. _FORTIFY_SOURCE vs our ASAN).
 config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"