lld: Fix incorrect physical address on self-referencing AT command.

When a section placement (AT) command references the section itself,
the physical address of the section in the ELF header was calculated
incorrectly due to alignment happening right after the location
pointer's value was captured.

The problem was diagnosed and the first version of the patch written
by Erick Reyes.

Obtained from: LLVM r322421 by Rafael Espindola