qemu: Exclude building on riscv32

Needs syscalls issues addressed. fails currently to build

../qemu-6.1.0/linux-user/syscall.c:659:1: error: use of undeclared identi
fier '__NR_nanosleep'; did you mean 'safe_nanosleep'?

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>