[Core] RISC-V toolchain and picolibc fixes (#15109)

* [Core] Fix RISC-V toolchain installation

The risc-v toolchain is only available on distributions based on Debian 11+
so we check for their availability before installing them.

* [Core] Fix heap symbols and syscalls for picolibc

picolibc internally uses __heap_start and __heap_end instead of the
defacto chibios linker script standard __heap_base__ and __heap_end__
therefore we introduce these symbols as an alias. Usually all memory
used within QMK is statically allocated, but some algorithms make usage
of malloc and friends.

Also the timeval struct is not defined by picolibc for syscalls, therefore it
is declared as stub.

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