19791: makefiles/arch/riscv.inc.mk: speed up toolchain detection r=benpicco a=maribu
### Contribution description
- Use a sane (a.k.a. simply expanded) variable for the `$(TARGET_ARCH)` instead of an insane (a.k.a. recursive expended) variable - The toolchain detection will now happen only once, rather than each and every time `$(TARGET_ARCH)` is referenced
- Use a single call to `which` rather than one per possible target triple
Fixes https://github.com/RIOT-OS/RIOT/issues/19788
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@posteo.net>
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