util/xcompile/xcompile: Allow to override `HOSTCC` variable

Currently `xcompile` generates `.xcompile` with the following at the
top.

	# platform agnostic and host tools
	IASL:=iasl
	HOSTCC:=gcc

The assignment `:=` doesn’t allow to override the variable. So use `?=`
instead so the host compiler can be passed to coreboot.

	HOSTCC=gcc-5 make

Note, that this is just a hack, as the existence of `gcc` is checked
beforehand.

Change-Id: Iebf3e43eb7eaffa7cf0efe97710d9feb3fe2a989
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/9457
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/xcompile/xcompile

@@ -63,7 +63,7 @@ fi
 cat <<EOF
 # platform agnostic and host tools
 IASL:=${IASL}
-HOSTCC:=${HOSTCC}
+HOSTCC?=${HOSTCC}
 
 EOF