shim 15.3 release candidate 2

Yay, another release candidate.  Here's the shortlog since rc1:

Javier Martinez Canillas (1):
      shim: Fix a NULL pointer dereference caused by start not being set

Peter Jones (2):
      Nerf an unnecessary string test on arm.
      arm and aarch64: fix some PE headers.

Steve McIntyre (2):
      Stop using EFI_PATH, and error out if people try to use it
      Fix up build of test code using gcc 8
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