flag-o-matic.eclass: whitelist -mstackrealign for 'strip-flags'

Allow '-mstackrealign' in ebuilds like glibc to force stack realignment.
That will allow running ancient (or foreign) binaries that don't
maintain 16-byte stack alignment. See bug #677852 for example.

Reported-by: Michael Huss
Closes: https://bugs.gentoo.org/677852
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

eclass/flag-o-matic.eclass

@@ -69,6 +69,13 @@ setup-allowed-flags() {
 		# gcc 4.9
 		-mno-avx512cd -mno-avx512er -mno-avx512f -mno-avx512pf -mno-sha
 	)
+
+	# Allow some safe individual flags. Should come along with the bug reference.
+	ALLOWED_FLAGS+=(
+		# Allow explicit stack realignment to run non-conformant
+		# binaries: bug #677852
+		-mstackrealign
+	)
 }
 
 # inverted filters for hardened compiler.  This is trying to unpick