Disable LLD_BOOTSTRAP under WITHOUT_CROSS_COMPILER

LLD is a cross toolchain component. It shouldn't be built when
requesting a build without building a cross compiler.

(CROSS_COMPILER is somewhat unfortunately named; in any case, lld
should be treated as GNU binutils here.)

Submitted by: Dan McGregor <dan.mcgregor at usask.ca>
MFC after: 1 week