[ELF] - Do not set st_size field of SHT_UNDEF symbols.

This fixes PR33598.

Size field for undefined symbols is not significant.
Setting it to fixed value, like zero, may be useful though.

For example when we have 2 DSO's, like in this PR, if lower level DSO may
change slightly (in part of some symbol's st_size)  and higher-level DSO is
rebuilt, then tools that monitoring checksum of high level DSO file can notice
it and trigger cascade of some other unnecessary actions. 
If we set st_size to zero, that can be avoided.

Differential revision: https://reviews.llvm.org/D34673

llvm-svn: 306526