Fix Mach-O unit tests breakage on Windows

Mach-O normalized file reader assumes that the entire file is aligned
to a large boundary. If the in-memory file is not aligned properly, it will
abort with an assertion failure in read32/read64. This patch forces the
in-memory file for the unit test to be aligned at 64-byte boundary.

I found these tests are failing on Windows, but theoretically they could
fail on other platform.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221508 91177308-0d34-0410-b5e6-96231b3b80d8