[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
[mach-o] Add support for interposing tuples section

Darwin uses two-level-namespace lookup for symbols which means the static
linker records where each symbol must be found at runtime.  Thus defining a
symbol in a dylib loaded earlier will not effect where symbols needed by
later dylibs will be found.  Instead overriding is done through a section
of type S_INTERPOSING which contains tuples of <interposer, interposee>.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221421 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Add a comment for SECREL relocation.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221423 91177308-0d34-0410-b5e6-96231b3b80d8
[mach-o] Add support for interposing tuples section

Darwin uses two-level-namespace lookup for symbols which means the static
linker records where each symbol must be found at runtime.  Thus defining a
symbol in a dylib loaded earlier will not effect where symbols needed by
later dylibs will be found.  Instead overriding is done through a section
of type S_INTERPOSING which contains tuples of <interposer, interposee>.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221424 91177308-0d34-0410-b5e6-96231b3b80d8
[mach-o] remove extra leading underscore on __bss and __got

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221425 91177308-0d34-0410-b5e6-96231b3b80d8