MC: Extract ELFObjectWriter's ELF writing functionality into an ELFWriter class. NFCI.
MC: Extract ELFObjectWriter's ELF writing functionality into an ELFWriter class. NFCI.

The idea is that we will be able to use this class to create multiple
files.

Differential Revision: https://reviews.llvm.org/D47048

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@332867 91177308-0d34-0410-b5e6-96231b3b80d8

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