[MinGW] Add an lld specific option for requesting to delay load libraries

With GNU tools, delayload is handled completely differently. (One
creates a specific delayload import library using dlltool and then
links against it instead of the normal import library.)

Instead of requiring using -Xlink=-delayload:lib.dll, we can provide
an lld specific option for this.

Differential Revision: https://reviews.llvm.org/D65728

llvm-svn: 367837