[llvm-objcopy] - Fix a st_name of the first symbol table entry.

Spec says about the first symbol table entry that index 0 both designates the first entry in the table
and serves as the undefined symbol index. It should have zero value.
Hence the first symbol table entry has no name. And so has to have a st_name == 0.
(http://refspecs.linuxbase.org/elf/gabi4+/ch4.symtab.html)

Currently, we do not emit zero value for the first symbol table entry.
That happens because we add empty strings to the string builder, which
for each such case adds a zero byte:
(https://github.com/llvm-mirror/llvm/blob/master/lib/MC/StringTableBuilder.cpp#L185)
After the string optimization performed it might return non zero indexes for the
empty string requested.

The patch fixes this issue for the case above and other sections with no names.

Differential revision: https://reviews.llvm.org/D59496

llvm-svn: 356739