Split Writer::createSections. NFC.
Split Writer::createSections. NFC.

The linker has to create __tls_get_addr, end and _end symbols.
Previously, these symbols were created in createSections().
But they are not actually related to creating output sections.
This patch moves code out of the function.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@256441 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld