Add checks for Spectre v2 mitigation (retpoline)

This adds checks for gcc and clang flags for mitigations for Spectre
variant 2, ie "retpoline".  It'll automatically enabled if the compiler
supports it as part of toolchain hardening flag.  ok djm@

configure.ac

@@ -163,6 +163,10 @@ if test "$GCC" = "yes" || test "$GCC" = "egcs"; then
 	OSSH_CHECK_CFLAG_COMPILE([-Wunused-result], [-Wno-unused-result])
 	OSSH_CHECK_CFLAG_COMPILE([-fno-strict-aliasing])
     if test "x$use_toolchain_hardening" = "x1"; then
+	OSSH_CHECK_CFLAG_COMPILE([-mfunction-return=thunk]) # gcc
+	OSSH_CHECK_CFLAG_COMPILE([-mindirect-branch=thunk]) # gcc
+	OSSH_CHECK_CFLAG_COMPILE([-mretpoline]) # clang
+	OSSH_CHECK_CFLAG_LINK([-z retpolineplt])
 	OSSH_CHECK_CFLAG_COMPILE([-D_FORTIFY_SOURCE=2])
 	OSSH_CHECK_LDFLAG_LINK([-Wl,-z,relro])
 	OSSH_CHECK_LDFLAG_LINK([-Wl,-z,now])