[Backtracing] Fix struct packing for PE-COFF definitions.
Due to unfortunate structure packing rules in C, `struct pe_symbol` ends up
being 20 bytes instead of 18. The upshot is that when presented with a PE-COFF
binary that contains symbols (noting that this is actually a spec violation),
the backtracer will crash.
Unfortunately we often generate such binaries. We shouldn’t crash, though, and
the fix is to get the correct size for `struct pe_symbol`.
rdar://171711254
