[PECOFF] Truncate long section name.

Executable files do not use a string table, so section names longer than 8
characters are not permitted. Long section names should just be truncated.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197470 91177308-0d34-0410-b5e6-96231b3b80d8