selftests: Set CC using CROSS_COMPILE once in lib.mk

This avoids repeating the logic in every Makefile. We mimic the
top-level Makefile and use $(CROSS_COMPILE)gcc.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/vm/Makefile

@@ -1,6 +1,5 @@
 # Makefile for vm selftests
 
-CC = $(CROSS_COMPILE)gcc
 CFLAGS = -Wall
 BINARIES = hugepage-mmap hugepage-shm map_hugetlb thuge-gen hugetlbfstest
 BINARIES += transhuge-stress