RFR(S): 7147740: add assertions to check stack alignment on VM entry from generated code (x64)

[Roland Westrelin]

>> Why you don't check it on windows_x86?
>
> Only because it's harder for me to build and test on windows and I assume testing on linux and solaris is sufficient to catch all problems.

I changed my mind and added support for windows:
http://cr.openjdk.java.net/~roland/7147740/webrev.01/

Given the windows ABI is somewhat different, maybe it's better to do the checks on windows as well.
I also fixed a typo in solaris_x86_64.il: _get_current_sp in the previous webrev returns rbp rather than rsp.

Roland.