Add getentropy for RTEMS

[thesummer]

RTEMS provides the getentropy function.
Use this for providing random data.

This PR enables the getentropy function for the RTEMS operating system to get random data.
It is exposed via libc (see rust-lang/libc#3975).

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[joboet]

Does RTEMS also have a suitable arc4random interface? If so, I'd prefer that, as we use that on nearly all other UNIXes.

Also I don't think the libc change has been released yet? That'll need to happen before this can be merged.

r? joboet

[thesummer]

@joboet I checked and the arc4random function is also available. I updated the PR accordingly. This currently also does not require further changes to libc.

However, I might try to still move the function declaration there.

[thesummer]

The changes in libc have been released tonight: https://github.com/rust-lang/libc/releases/tag/0.2.160

@joboet My suggestion would be to merge this PR with arc4random_buf defined here in std. I will add the definition to libc, so that it will become part of 0.2.161 and change it here afterwards. Alternatively, we can go back to the getentropy version and move to arc4random with the next libc release. Any preferences?

My goal is to get a first compiling version of the RTEMS tier 3 port into the mainline, so I can setup a local CI to track changes/regressions. Currently I have to manually monitor the different repos which is not ideal.