Simplify getStringTableIndex.

The description in the ELF spec is just

---------------------------
If the section name string table section index is greater than or
equal to SHN_LORESERVE (0xff00), this member has the value SHN_XINDEX
(0xffff) and the actual index of the section name string table section
is contained in the sh_link field of the section header at index 0.
---------------------------

So we only have to check for it being SHN_XINDEX. Also, sh_link is
always 32 bits, so don't return an uintX_t.

llvm-svn: 285747