kernel-2.eclass: use $(CC) as HOSTCC

Before the change HOSTCC always used gcc. This was
detected by Agostino on linux-headers package.

After the change HOSTCC uses user-specified CC
(or BUILD_CC). Tested on native linux-headers
and on cross-*/linux-headers.

Reported-by: Agostino Sarubbo
https://bugs.gentoo.org/725878
Signed-off-by: Sergei Trofimovich <[email protected]>

eclass/kernel-2.eclass

@@ -712,6 +712,7 @@ env_setup_xmakeopts() {
 	elif type -p ${CHOST}-ar > /dev/null ; then
 		xmakeopts="${xmakeopts} CROSS_COMPILE=${CHOST}-"
 	fi
+	xmakeopts="${xmakeopts} HOSTCC=$(tc-getBUILD_CC)"
 	export xmakeopts
 }