Add `efi_rng` opt-in backend #1868

Job	Run time
Tier 1 (ubuntu-24.04, nightly)	16s
Cross (aarch64-unknown-linux-gnu)	24s
Tier 1 (ubuntu-24.04, beta)	16s
Cross (powerpc-unknown-linux-gnu)	25s
Tier 1 (ubuntu-24.04, stable)	14s
FreeBSD VM	2m 18s
Tier 1 (ubuntu-24.04, 1.63)	22s
Tier 1 (windows-2022, nightly)	40s
iOS Simulator	3m 58s
Tier 1 (windows-2022, beta)	47s
Linux (x86_64-unknown-linux-musl)	27s
Tier 1 (windows-2022, stable)	48s
Linux (i686-unknown-linux-musl)	21s
OpenBSD VM	2m 10s
Tier 1 (windows-2022, 1.63)	2m 48s
Tier 1 (macos-14, stable)	27s
Sanitizer	30s
Windows (stable-x86_64-gnu)	53s
Windows 7 (on Windows 10)	1m 29s
Windows (stable-i686-gnu)	49s
Windows (stable-i686-msvc)	57s
WASI	21s
Web	1m 0s
Web with Atomics	1m 20s
	24m 0s

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