emitbc: Fix structure field alignment issue.

dummy_data field is accessed as uint value (e.g.
in emit_write_bytecode_byte_ptr), but is not aligned as such, which causes
bus errors or incorrect behavior on any arch requiring strictly aligned
data (ARM pre-v7, MIPS, etc, etc).
py: Add generic helper to align a pointer.
run-tests: Add option to write CPython's test results to .exp files.

Mostly to run testsuite on targets which doesn't have CPython.