Add Apache NuttX support

[kay-lambdadelta]

This PR adds the ability to use getrandom on NuttX.

Supporting this target is fairly uninteresting as it has support for a linux-style getrandom which is supported via libc

This does however, require a bump for the version of libc this crate uses (to 0.2.176 for errno support)

[newpavlov]

Would it be possible to test the target in out CI using build-only job?

[kay-lambdadelta]

Would it be possible to test the target in out CI using build-only job?

A build-only test shouldn't require anything more than a nightly compiler. I would suggest pinning to a nightly version like nightly-2026-08-09, as rust NuttX support is pretty alpha and it breaks sometimes. Right now the riscv and the arm variants of the target should work.