Makefile: Fix error evaluating OPENSBI_CC_XLEN

We get following error evaluating OPENSBI_CC_XLEN on
Ubuntu-18.04:
/bin/sh: 1: Bad substitution

This patch fixes above error by using "awk" in string
assigned to OPENSBI_CC_XLEN and it also moves "Setup
compilation commands" before OPENSBI_CC_XLEN.

Signed-off-by: Anup Patel <anup.patel@wdc.com>