lib: sbi: Speed-up sbi_printf() and friends using nputs()

The sbi_printf() is slow for semihosting because it prints one
character at a time. To speed-up sbi_printf() for semihosting,
we use a temporary buffer and nputs().

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>