7005007: Refine use of ALT_COMPILER_PATH to avoid conflict with JPRT usage

Summary: Check for CROSS_COMPILE_ARCH being set as an indicator to use ALT_COMPILER_PATH
Reviewed-by: acorn, ohair

make/linux/makefiles/build_vm_def.sh

 #!/bin/sh
 
 # If we're cross compiling use that path for nm
-if [ "$ALT_COMPILER_PATH" != "" ]; then 
+if [ "$CROSS_COMPILE_ARCH" != "" ]; then 
 NM=$ALT_COMPILER_PATH/nm
 else
 NM=nm

make/linux/makefiles/gcc.make

@@ -25,7 +25,9 @@
 #------------------------------------------------------------------------
 # CC, CPP & AS
 
-ifdef ALT_COMPILER_PATH
+# When cross-compiling the ALT_COMPILER_PATH points
+# to the cross-compilation toolset
+ifdef CROSS_COMPILE_ARCH
 CPP = $(ALT_COMPILER_PATH)/g++
 CC  = $(ALT_COMPILER_PATH)/gcc
 else