Fix codegen_test for long multiplication.

It seems like clang (that we use on the host) was *very* forgiving
with this broken test: the code generated for for MulLong used ebx
but this is a callee-save register in C but not ART. Also, the test
was not properly written for handling longs, so it was taking
unitialized stack entries.

GCC on target is not as forgiving.

Change-Id: I5d7a962f8a72b3ce407dce50ca50b4ffc690c99e