[compiler] Prevent empty weak function stubs from being removed

[compiler] Prevent empty weak function stubs from being removed

Even with the noinline specifier added by commit 1a260f8, gcc may skip
calls to non-inlinable functions that it knows have no side
effects. This caused the get_cached_dhcpack() call in start_dhcp(),
the weak stub of which has no code in its body, to be removed,
preventing cached DHCP from working.

Fix by adding a __keepme macro to compiler.h expanding to asm(""), as
recommended by gcc's info page, and using it in the weak stub for
get_cached_dhcpack().

Reported-by: Aaron Brooks <aaron@brooks1.net>
Tested-by: Aaron Brooks <aaron@brooks1.net>
Signed-off-by: Joshua Oreman <oremanj@rwcr.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>