musl: enable getrandom on all musl platforms

[tamird]

Description

The existing bindings were added in #1399 and limited to targets where
rustc used musl version >= 1.1.20 which was not all musl targets at that
time. Since rust-lang/rust#107129 all musl
targets use musl 1.2.3. Hence, move the binding to the module root so it
is available for all musl targets.

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[rustbot]

r? @tgross35

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[tgross35]

@rustbot label +stable-nominated