[ELF] - Allow relocation to a weak undefined symbol when -z notext is given.

Previously we failed to resolve them when produced executables:
"relocation R_X86_64_32 cannot be used against shared object; recompile with -fPIC"

Patch fixes it so that we resolve them to 0 for executables. 
And for -shared case we still should produce the relocation.

This finishes fixing PR35720.

DIfferential revision: https://reviews.llvm.org/D41551

llvm-svn: 321473