[linker] Add safe weak symbol macros

[linker] Add safe weak symbol macros

Weak symbols are a useful tool in eliminating unnecessary dependencies
between object files, but they are somewhat dangerous because one must
remember to test the weak symbol against NULL before using it. To
rectify that, add macros for declaring weak functions that will return
a default value inline if the file defining them is not available at
link time.

Signed-off-by: Marty Connor <mdc@etherboot.org>