Merge RIOT-OS#19791

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 RIOT-OS#19788


Co-authored-by: Marian Buschsieweke <marian.buschsieweke@posteo.net>

makefiles/arch/riscv.inc.mk

@@ -35,10 +35,15 @@ TARGET_ARCH_RISCV ?= \
     $(subst -gcc,,\
       $(notdir \
         $(word 1,\
-          $(foreach triple,$(_TRIPLES_TO_TEST),$(shell which $(triple)-gcc 2> /dev/null))))))
+          $(shell which $(addsuffix -gcc,$(_TRIPLES_TO_TEST)) 2> /dev/null)))))
 
 TARGET_ARCH ?= $(TARGET_ARCH_RISCV)
 
+# Convert to a simply expanded variable here, as a recursively expended
+# variable would result in detecting the toolchain each and every time again the
+# toolchain is referenced.
+TARGET_ARCH := $(TARGET_ARCH)
+
 ifeq (,$(TARGET_ARCH))
   $(error No RISC-V toolchain detected. Make sure a RISC-V toolchain is installed.)
 endif