LinkerScript: Add -T <scriptfile> option

GNU LD has an option named -T/--script which allows a user to specify
a linker script to be used [1].  LLD already accepts linker scripts
without this option, but the option is widely used.  Therefore it is
best to support it in LLD as well.

[1] https://sourceware.org/binutils/docs/ld/Options.html#Options

llvm-svn: 232183