[PECOFF] Fix SECREL relocations.

SECREL relocation's value is the offset to the beginning of the section.
Because of the off-by-one error, if a SECREL relocation target is at the
beginning of a section, it got wrong value.

Added a test that would have caught this.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221420 91177308-0d34-0410-b5e6-96231b3b80d8