Add ENTRY_NB() macro for doing an ASM function entry without setting

Add ENTRY_NB() macro for doing an ASM function entry without setting
the binding to global (NB == "no binding"), as clang 13 is now
warning about changing the binding from global to weak.

This first pass does amd64 and sparc64 and pulls DEFS.h out of the
per-arch directory to a common directory; others to follow

ok kettenis@