makefiles: Pass explicit float mode to clang
makefiles: Pass explicit float mode to clang

The assumption that clang defaults to hard-float on ARM architectures
does not hold on Debian systems where [set-correct-float-abi].
As setting the flags that are implied on most systems does no harm, the
flags for the selected hard- or soft float behavior are now set
independently of the toolchain.

[set-correct-float-abi]: https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/-/blob/10/debian/patches/26-set-correct-float-abi.diff

Closes: https://github.com/RIOT-OS/RIOT/issues/15026

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