[mach-o] Add support for -sectalign option

The -sectalign option is used to increase the alignment required for a section.
It required some reworking of how the __TEXT segment is laid out because that
segment also contains the mach_header and load commands. And the size of load
commands depend on the number of segments, sections, and dependent dylibs used.

Using this option will simplify some future test cases because the final
address of code can be pinned down, making tests of its content easier.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@214268 91177308-0d34-0410-b5e6-96231b3b80d8