[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>

src/net/udp/dhcp.c

@@ -1389,7 +1389,7 @@ static struct sockaddr dhcp_peer = {
 /**
  * Get cached DHCPACK where none exists
  */
-__weak void get_cached_dhcpack ( void ) {}
+__weak void get_cached_dhcpack ( void ) { __keepme }
 
 /**
  * Start DHCP state machine on a network device