Ignore functions referencing symbol at 0x0.

Summary:
Binary code could be weird. It could include calls to address 0 and
reference data at 0 (e.g. with lea on x86). LLVM JIT fatals
while resolving relocations against symbols at address 0x0. For now
we will stop emitting such code, i.e. we'll skip functions.

(cherry picked from FBD28109837)