[RuntimeDyld][MachO] Fix assertion in encodeAddend, add missing directive to

test case.

r326290 fixed the assertion for decodeAddend, but not encodeAddend. The
regression test failed to catch this because it was missing the
subsections_via_symbols flag, so the desired relocation was not applied.

This patch also fixes the formatting of the assertion from r326290.

llvm-svn: 326406