[PECOFF] Add /section option.

/section command line option is to set/reset attributes of the Characteristics
field in the section header. You can set non-default values with this option.
You can make .data section executable with this, for example.

This patch implements the parser of the command line option. The code to use
the parsed values will be committed in a separate patch.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194133 91177308-0d34-0410-b5e6-96231b3b80d8