cmake: linker: lld: Do not link libc when minimal libc is used

When LLVM linker is used, the toolchain-provided C standard library is
always linked, even when the Zephyr minimal libc is selected, because `c`
is unconditionally specified in `link_order_library`, which causes `-lc` to
be specified in the linker flags.

This commit adds a check to ensure that `-lc` is not specified when the
Zephyr minimal libc is selected because it is a standalone C standard
library and it does not make sense to link two C standard libraries at
once.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>