ukify: Prefer using llvm-objcopy instead of objcopy

ukify: Prefer using llvm-objcopy instead of objcopy

llvm-objcopy works on stubs built for foreign architectures whereas
objcopy doesn't so let's prefer using llvm-objcopy instead of objcopy.

llvm-objcopy automatically sets the virtual address and doesn't provide
an option to set it manually so we only add --change-section-vma when
using objcopy

The default section flags differ between llvm-objcopy and objcopy
so we add a default for the section flags so we make sure all sections
are read-only data unless specified otherwise.