build: Fix not allowing the host tools to be crosscompiled

There are situations that the host gcc is not able to generate x86
binaries, in those cases, the user should point HOSTCC to a binary able
to generate the correct type of binaries.

This also allows './scripts/sanitycheck' to be run in systems where the
build tools are prefixed by default.

Change-Id: Id1fc247f8a820e19bb3d4e8119634f8300e7ebac
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>

Makefile

@@ -267,8 +267,8 @@ CONFIG_SHELL := $(shell if [ -x "$$BASH" ]; then echo $$BASH; \
 	  else if [ -x /bin/bash ]; then echo /bin/bash; \
 	  else echo sh; fi ; fi)
 
-HOSTCC       = gcc
-HOSTCXX      = g++
+HOSTCC       ?= gcc
+HOSTCXX      ?= g++
 HOSTCFLAGS   = -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer -std=gnu89
 HOSTCXXFLAGS = -O2