[llvm-objcopy][ELF] -O binary: use LMA instead of sh_offset to decide where to write...

[llvm-objcopy][ELF] -O binary: use LMA instead of sh_offset to decide where to write section contents

.text sh_address=0x1000 sh_offset=0x1000
.data sh_address=0x3000 sh_offset=0x2000

In an objcopy -O binary output, the distance between two sections equal
their LMA differences (0x3000-0x1000), instead of their sh_offset
differences (0x2000-0x1000). This patch changes our behavior to match
GNU.

This rule gets more complex when the containing PT_LOAD has
p_vaddr!=p_paddr. GNU objcopy essentially computes
sh_offset-p_offset+p_paddr for each candidate section, and removes the
gap before the first address.

Added tests to binary-paddr.test to catch the compatibility problem.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D71035