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.
unix: Allow to disable MICROPY_EMIT_X64 from commandline.

emitnative in particular requires nlr_* to be real functions, so doesn't
compile with MICROPY_NLR_SETJMP=1.
test: Add run-tests-exp.sh, script to run testsuite with only sh dependency.

This script uses expected test results as generated by run-tests --write-exp,
and requires only standard unix shell funtionality (no bash). It is useful
to run testsuite on embedded systems, where there's no CPython and Bash.
unix: file: Implement .flush() method.

This method apparently should be part of stream interface.
stream: Factor out mp_stream_write() method to write a memstring to stream.
unix: file: No fsync() on Windows.
Merge https://github.com/micropython/micropython