[coreboot] lots of compilation failures in abuild on x86-64

[Cristi Magherusan]

Hello,

I tried to run abuild on my x86-64 box and I got compilation errors like
the following, for almost all the targets:

Compiling image on 1 cpu .. FAILED after 17s! Log excerpt:
(.text+0x101db): undefined reference to `__divdi3'

The fix is pretty simple, but I have no idea if it's acceptable:
Use gcc-multilib, and add -lgcc to the LDFLAGS for all the broken
targets.

I will provide soon a log with all the broken boards.

Regards,
Cristi