llvm-readobj: add support to dump (COFF) directives

PE/COFF has a special section (.drectve) which can be used to pass options to
the linker (similar to LC_LINKER_OPTION).  Add support to llvm-readobj to print
the contents of the section for tests.
MC: add support for -aligncomm GNU extension

The GNU linker supports an -aligncomm directive that allows for power-of-2
alignment of common data.  Add support to emit this directive.