[RuntimeDyld] Skip relocations for external symbols with 64-bit address ~0ULL.

[RuntimeDyld] Skip relocations for external symbols with 64-bit address ~0ULL.

Requested by Eugene Rozenfeld of the LLILC team, this feature allows JIT
clients to skip relocations for selected external symbols by returning ~0ULL
from their symbol resolver. If this value is returned for a given symbol,
RuntimeDyld will skip all relocations for that symbol. The client will be
responsible for applying the skipped relocations manually before the code
is executed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241383 91177308-0d34-0410-b5e6-96231b3b80d8