Makefile: Fix verbose builds on Windows

Commit <ee1ba6d4> ("Makefile: Support totally quiet output with -s")
broke verbose (V=1) builds on Windows. This patch fixes it by adding
helpers to silence echo prints in a OS-dependent way.

Change-Id: I24669150457516e9fb34fa32fa103398efe8082d
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>