[LIT][Darwin] Change %ld64 to be prefixed with DYLD_INSERT_LIBRARIES

Followup from r277778, after Mehdi's comments.

Expand %ld64 to perform the necessary preload instead, that way new
tests do not need to worry about setting up DYLD_INSERT_LIBRARIES
themselves.

rdar://problem/24300926

llvm-svn: 277788