[libunwind] Fix wrong end argument passed to decodeEHHdr() (#68813)

All but one callsite were actually passing start+length arguments.
This should not have any functional change since the end argument is
almost always ignored.
I noticed this while debugging some incorrect error messages being
printed while running the testsuite baremetal (using binaries that did
not have a valid eh_frame_hdr section): the tests print
`libunwind: unsupported .eh_frame_hdr version: 20 at
https://github.com/arichardson/upstream-llvm-project/commit/8000d308146ebf49cb364cb600e28a0a42e22c83`
because
libunwind is reading nonsense data for .eh_frame_hdr.