Rollup merge of #131774 - thesummer:rtems-add-getentropy, r=joboet

Add getentropy for RTEMS

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 https://github.com/rust-lang/libc/pull/3975).

library/std/src/random.rs

@@ -40,6 +40,7 @@
 /// Horizon                | `getrandom` shim
 /// Hurd, L4Re, QNX        | `/dev/urandom`
 /// Redox                  | `/scheme/rand`
+/// RTEMS                  | [`arc4random_buf`](https://docs.rtems.org/branches/master/bsp-howto/getentropy.html)
 /// SGX                    | [`rdrand`](https://en.wikipedia.org/wiki/RDRAND)
 /// SOLID                  | `SOLID_RNG_SampleRandomBytes`
 /// TEEOS                  | `TEE_GenerateRandom`

library/std/src/sys/random/arc4random.rs

@@ -12,6 +12,7 @@
 #[cfg(not(any(
     target_os = "haiku",
     target_os = "illumos",
+    target_os = "rtems",
     target_os = "solaris",
     target_os = "vita",
 )))]
@@ -21,6 +22,7 @@
 #[cfg(any(
     target_os = "haiku", // See https://git.haiku-os.org/haiku/tree/headers/compatibility/bsd/stdlib.h
     target_os = "illumos", // See https://www.illumos.org/man/3C/arc4random
+    target_os = "rtems", // See https://docs.rtems.org/branches/master/bsp-howto/getentropy.html
     target_os = "solaris", // See https://docs.oracle.com/cd/E88353_01/html/E37843/arc4random-3c.html
     target_os = "vita", // See https://github.com/vitasdk/newlib/blob/b89e5bc183b516945f9ee07eef483ecb916e45ff/newlib/libc/include/stdlib.h#L74
 ))]

library/std/src/sys/random/mod.rs

@@ -17,6 +17,7 @@
         target_os = "illumos",
         target_os = "netbsd",
         target_os = "openbsd",
+        target_os = "rtems",
         target_os = "solaris",
         target_os = "vita",
     ))] {