RFR (S): 7172922: export_ makefile targets do not work unless all supported variants are built

[Christian Thalinger]

http://cr.openjdk.java.net/~twisti/7172922

7172922: export_ makefile targets do not work unless all supported variants are built
Reviewed-by:

GEN_DIR can be overwritten by other configurations if multiple JVM_VARIANT_*s are defined. The fix is to use the *_BASE_DIRs directly to install the correct files.

make/Makefile