arch/x86: Add a common romstage entry

It might be possible to have this used for more than x86, but that
will be for a later commit.

Change-Id: I4968364a95b5c69c21d3915d302d23e6f1ca182f
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55067
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

src/include/romstage_common.h

@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef ROMSTAGE_COMMON_H
+#define ROMSTAGE_COMMON_H
+
+void __noreturn romstage_main(void);
+
+#endif	/* ROMSTAGE_COMMON_H */