handling of __attribute__((weak)) in interface inference

[ckaestne]

With this attribute the symbol is emitted as a weak symbol that may be overridden by other exports.
http://gcc.gnu.org/onlinedocs/gcc-3.2/gcc/Function-Attributes.html

A notion of weak symbols is not supported yet by our interface mechanisms

Occurs in Linux when linking together mach_reboot_fixups in files arch/x86/kernel/reboot_fixups_32 and arch/x86/kernel/reboot