[ELF] - Implemented --retain-symbols-file option

[ELF] - Implemented --retain-symbols-file option

--retain-symbols-file=filename
Retain only the symbols listed in the file filename, discarding all others.
filename is simply a flat file, with one symbol name per line. This option
is especially useful in environments (such as VxWorks) where a large global
symbol table is accumulated gradually, to conserve run-time memory.

Note: though documentation says "--retain-symbols-file does not discard
undefined symbols, or symbols needed for relocations.", both bfd and gold
do that, and this patch too, like testcase show.

Differential revision: https://reviews.llvm.org/D27716

llvm-svn: 290122