Assume getrandom on Linux

[ysbaddaden]

The getrandom(2) syscall was added in 2017 and at the time we couldn't expect the glibc 2.25 to be widely available, but we're in 2024 now, and even Ubuntu 18.04 LTS that is now EOL had a compatible glibc release (2.27). I assume musl-libc also added the symbol at the same time.

We can simplify the implementation to assume getrandom is available, which avoids the initial check, initialization and fallback to urandom. We still fallback to urandom at compile time when targeting android api level < 28 (we support 24+).

An issue is that executables will now expect glibc 2.25+ (for example), though the interpreter already did.

Related to #15034.

[straight-shoota]

https://crystal-lang.org/reference/1.13/syntax_and_semantics/platform_support.html list already as a requirement GNU libc 2.26+. However, I have no idea what's the driving factor 🙈 I added the version information in crystal-lang/crystal-book#678, but I don't recall where the number comes from.

[ysbaddaden]

We also expect kernel 2.6.18 to be compatible, but getrandom was added in 3.17 which means it depends on how the libc symbol is implemented —does it fallback to urandom, does it fail?