[llvm-objcopy] -O binary: consider SHT_NOBITS sections to be empty

This is consistent with BFD objcopy.

Previously llvm objcopy would allocate space for SHT_NOBITS sections
often resulting in enormous binary files.

New test case (binary-paddr.test %t6).

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D95569