[PECOFF] Do not allow space to separate command line option and its value.

We used to support both Windows and Unix style command line options. In Windows
style, an option and its value are separated by ":" (colon). In Unix, separator
is a space. Accepting both styles were convenient, but we can no longer allow
Unix style because I found that can be ambiguous.

For example, /nodefaultlib option takes an optional argument. In Windows style
it's going to be something like "/nodefaultlib:foo". There's no ambiguity what
"foo" means. However, if the option is "/nodefaultlib foo", "foo" can be
interpreted either an optional argument for "/nodefaultlib" or an input file
"foo.obj". We should just stop accepting the non-standard command line style.

llvm-svn: 191247