For RISC-V and Cortex-M-not-3, triples are known and have worked in some
configuration, but do not work at the moment and stay disabled until the
reference platforms (native, M3) have been established well.
This requires -nostartfiles to be only passed to the linker, not the
compiler, as it is a linker flag and passing it to the compiler causes a
clang warning to be emitted.
Additionally, clang does not seem to support `-mcmodel=medlow` and
`-msmall-data-limit=8` but these options do not seem strictly necessary
to me anyhow thus they are deactivated conditionally when using clang.
Use riscv-none-elf instead of legacy riscv-none-embed as target triplet for
RISC-V development. However, if ricsv-none-elf is not present, try
riscv64-unknown-elf and riscv-none-embed instead. If the legacy riscv-none-embed
is used, a warning is printed.
Add `TARGET_ARCH_<ARCH>` for each architecture (e.g. `TARGET_ARCH_CORTEX` for
Cortex M) to allow users to overwrite the target triple for a specific arch
from ~/.profile or ~/.bashrc (or the like) without overwriting it for all others
as well.
