Fix Linux getrandom failure in interpreted code (#15035)

Whereas the syscall returns the negative of the `Errno` value on failure, the `LibC` function returns -1 and then sets `Errno.value`. Crystal always assumes the former:

```cr
 err = Errno.new(-read_bytes.to_i) 
 if err.in?(Errno::EINTR, Errno::EAGAIN) 
   ::Fiber.yield 
 else 
   raise RuntimeError.from_os_error("getrandom", err) 
 end 
```

https://github.com/crystal-lang/crystal/blob/cde543a762d56324e1d1261154d767c84db1ebc1/src/crystal/system/unix/getrandom.cr#L105-L110

As `EPERM` equals 1 on Linux, _all_ failures are treated like `EPERM` in interpreted code, even though `EPERM` itself is not listed as an error for [`getrandom(2)`](https://man7.org/linux/man-pages/man2/getrandom.2.html), hence the "Operation not permitted". The same can probably happen on other Linux distros if you run out of entropy.

src/crystal/system/unix/getrandom.cr

@@ -13,7 +13,10 @@ require "./syscall"
 
     # TODO: Implement syscall for interpreter
     def self.getrandom(buf : UInt8*, buflen : LibC::SizeT, flags : UInt32) : LibC::SSizeT
-      LibC.getrandom(buf, buflen, flags)
+      # the syscall returns the negative of errno directly, the C function
+      # doesn't, so we mimic the syscall behavior
+      read_bytes = LibC.getrandom(buf, buflen, flags)
+      read_bytes >= 0 ? read_bytes : LibC::SSizeT.new(-Errno.value.value)
     end
   end
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