Support linker scripts for $(ld_targets)

Like 'ccld', the 'ld' function understands when linker scripts should be
added to the 'ld' invocation.

However, the LDFLAGS += -T... in the 'ccld' and 'ld' build functions just
bit me: since LDFLAGS is exported, the linker script is carried to each new
invocation of 'ld' which is not desirable. I now use the LDS variable which
is not exported.