[RuntimeDyld] Allow multi-line rtdyld-check and jitlink-check expressions.

This patch allows rtdyld-check / jitlink-check expressions to be extended over
multiple lines by terminating each line with a '\'. E.g.

  # llvm-rtdyld: *{8}X = \
  # llvm-rtdyld:   Y
  X:
    .quad Y

This will be used to break up some long lines in upcoming test cases.