[build] Mark weak functions noinline

[build] Mark weak functions noinline

Weak functions whose visibility is hidden may be inlined due to a bug
in GCC.  Explicitly mark weak functions noinline to work around the
problem.

This makes the PXE_MENU config option work again, the PXE boot menu
was never being called because the compiler inlined a weak stub
function.

The GCC bug was identified and fixed by Richard Sandiford
<rdsandiford@googlemail.com> but in the meantime iPXE needs to
implement a workaround.

Reported-by: Steve Jones <steve@squaregoldfish.co.uk>
Reported-by: Shao Miller <shao.miller@yrdsb.edu.on.ca>
Suggested-by: Joshua Oreman <oremanj@rwcr.net>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>