ArmPlatformPkg/DS-5: fix 64-bit PE/COFF header parsing bug

ArmPlatformPkg/DS-5: fix 64-bit PE/COFF header parsing bug

The 64-bit version of the DS-5 debug script that retrieves the debug file
path from the PE/COFF image in memory assumes that the PE/COFF header is
packed, and that the debug directory entry in the optional header appears
at a fixed offset into the file. This is no longer true, now that we pad
between the file header and the PE header if the section alignment exceeds
the size of the header (which may be the case when the module contains a
vector table or small model code, which requires 2 KB or 4 KB section
alignment, respectively), to allow this padding to be emitted if the image
is subsequently converted to TE format.

So replace the fixed offset with a dereference of the appropriate header
field.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reported-by: Vladimir Olovyannikov <vladimir.olovyannikov@broadcom.com>