lib: sbi: Fix printf handling of long long

Read long long arguments directly using va_arg. Remove original hack for
RV32 that read a long long arg as two long args.

This un-breaks the case on RV64 where e.g. the long long is followed by
an odd number of ints:

    sbi_printf("%d %lld", (int) 1, (long long) 2LL);

Also remove the acnt variable, which is now unused.

Signed-off-by: dramforever <dramforever@live.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>