[llvm-libtool-darwin] Parse and ignore -syslibroot

cctools libtool allows you to link dynamic libraries by passing through
a number of arguments to ld64. Because of this the default arguments
libtool receives in Xcode contains arguments that only matter in that
case. This change ignores this argument, at least until we ever support
that dynamic use case, so that you can use llvm-libtool-darwin as a
drop-in replacement in Xcode for cctools libtool. There are more
arguments we could ignore for this case, but we can probably add those
as the use case comes up.

Differential Revision: https://reviews.llvm.org/D134309