Simplify the logic in ELFObjectWriter::isInSymtab. NFC.
Simplify the logic in ELFObjectWriter::isInSymtab. NFC.

_GLOBAL_OFFSET_TABLE_ is not magical and we can now directly check for a
symbol never getting an explicit binding.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@238978 91177308-0d34-0410-b5e6-96231b3b80d8

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