tools, build: Add RISC-V to HOSTARCH parsing

Add RISC-V to the HOSTARCH parsing, so that ARCH is "riscv", and not
"riscv32" or "riscv64".

This affects the perf and libbpf builds, so that arch specific
includes are correctly picked up for RISC-V.

Signed-off-by: Björn Töpel <bjorn@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211028161057.520552-3-bjorn@kernel.org