Add new compiler hardening flags.

Add -fzero-call-used-regs and -ftrivial-auto-var-init to the list of
compiler hardening flags that configure checks for.  These are supported
by clang and gcc, and make ROP gadgets less useful and mitigate
stack-based infoleaks respectively.  ok djm@

configure.ac

@@ -190,6 +190,8 @@ if test "$GCC" = "yes" || test "$GCC" = "egcs"; then
 	# actually links. The test program compiled/linked includes a number
 	# of integer operations that should exercise this.
 	OSSH_CHECK_CFLAG_LINK([-ftrapv])
+	OSSH_CHECK_CFLAG_COMPILE([-fzero-call-used-regs=all])
+	OSSH_CHECK_CFLAG_COMPILE([-ftrivial-auto-var-init=zero])
     fi
 	AC_MSG_CHECKING([gcc version])
 	GCC_VER=`$CC -v 2>&1 | $AWK '/gcc version /{print $3}'`