use nodejs crypto api when building with emscripten

[jaromil]

Adds simple support to use the crypto api provided by nodejs when cross-building using emscripten.

[dsprenkels]

I propose the following patches (and pushed them to jaromil:emscripten-1).

• Changed the location of the function (to keep platform order from mostly used to least used).
• Pulled #include's to top of file.
• Changed function signature to static int.
• So I found that it may not even be necesarry to use a temporary buffer for in between. (So we do not need _malloc() and #include <stdlib.h>. Also, I added some error handling, because crypto.randomBytes mentions:

  An error will be thrown if there is a problem generating the bytes.

Would you agree to the proposed changes? Did I miss something?

[jaromil]

Much better now. I realise I won't be able on the short term to make an emscripten compatible test to work in the CI, but if you like to merge it anyway go ahead.

[dsprenkels]

Way ahead of ya. ;)

Thanks a lot for your contribution!