Avoid possible crash within libsanitizer.

We've successfully used libsanitizer for awhile with the undefined
and alignment sanitizers, but with some other sanitizers (at least
thread and hwaddress) it crashes due to internal recursion before
it's fully initialized itself.  It turns out that that's due to the
"__ubsan_default_options" hack installed by commit f686ae82f, and we
can fix it by ensuring that __ubsan_default_options is built without
any sanitizer instrumentation hooks.

Reported-by: Emmanuel Sibi <emmanuelsibi.mec@gmail.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Diagnosed-by: Emmanuel Sibi <emmanuelsibi.mec@gmail.com>
Fix-suggested-by: Jacob Champion <jacob.champion@enterprisedb.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/F7543B04-E56C-4D68-A040-B14CCBAD38F1@gmail.com
Discussion: https://postgr.es/m/dbf77bf7-6e54-ed8a-c4ae-d196eeb664ce@gmail.com
Backpatch-through: 16

src/backend/main/main.c

@@ -427,20 +427,29 @@ check_root(const char *progname)
 /*
  * At least on linux, set_ps_display() breaks /proc/$pid/environ. The
  * sanitizer library uses /proc/$pid/environ to implement getenv() as it wants
- * to work independent of libc. When just using undefined and alignment
+ * to work independent of libc. Depending on which sanitizers are enabled,
- * sanitizers, the sanitizer library is only initialized when the first error
+ * the sanitizer library may not get initialized until after we've called
- * occurs, by which time we've often already called set_ps_display(),
+ * set_ps_display(), preventing the sanitizer from seeing environment-supplied
- * preventing the sanitizer libraries from seeing the options.
+ * options.
  *
  * We can work around that by defining __ubsan_default_options, a weak symbol
  * libsanitizer uses to get defaults from the application, and return
  * getenv("UBSAN_OPTIONS"). But only if main already was reached, so that we
  * don't end up relying on a not-yet-working getenv().
  *
+ * On the other hand, with different sanitizers enabled, libsanitizer can
+ * call this so early that it's not fully initialized itself, resulting in
+ * recursion and a core dump within libsanitizer.  To prevent that, ensure
+ * that this function is built without any sanitizer callbacks in it.
+ *
  * As this function won't get called when not running a sanitizer, it doesn't
  * seem necessary to only compile it conditionally.
  */
 const char *__ubsan_default_options(void);
+
+#if __has_attribute(disable_sanitizer_instrumentation)
+__attribute__((disable_sanitizer_instrumentation))
+#endif
 const char *
 __ubsan_default_options(void)
 {