cmake: llvm: arm: Remove '--specs' when building with clang for arm

Currently, clang produces a warning that the '--specs=nosys.specs' argument
is unused. Remove this flag to fix the warning as clang will generally not
honor '--specs' when targeting arm baremetal.

While this flag has been present for some time, I think this should be safe
to do. For background, clang does not seem to handle '--specs' [1] besides
possibly passing it through to GCC if GCC is used as the driver for
linking. However, whether GCC will be used for linking as a fallback
depends on the "Toolchain" [1] clang uses internally, which in turn depends
on the triple. For arm/thumb baremetal triples, the Toolchain clang uses
will not fall back to GCC to drive linking, so '--specs' will never be used
here.

I believe this behavior in clang is fairly longstanding as well (since
~2017/LLVM 5 [2]). While there isn't a minimum required clang version for
Zephyr, Zephyr currently requires lld >= 14.0.0. So, I don't think removing
this flag should impact current users (besides preventing the warning).

[1] https://clang.llvm.org/docs/DriverInternals.html
[2] https://reviews.llvm.org/D33259



Signed-off-by: Jonathon Penix <jpenix@quicinc.com>