Add `efi_rng` opt-in backend #1823

Job	Run time
iOS Simulator	3m 43s
Tier 1 (ubuntu-24.04, nightly)	22s
Windows 7 (on Windows 10)	1m 51s
Linux (x86_64-unknown-linux-musl)	12s
Tier 1 (ubuntu-24.04, beta)	19s
Linux (i686-unknown-linux-musl)	16s
Tier 1 (ubuntu-24.04, stable)	32s
Cross (aarch64-unknown-linux-gnu)	36s
FreeBSD VM	2m 21s
Tier 1 (ubuntu-24.04, 1.63)	29s
Cross (powerpc-unknown-linux-gnu)	24s
OpenBSD VM	2m 11s
Tier 1 (windows-2022, nightly)	56s
Tier 1 (windows-2022, beta)	43s
Sanitizer	33s
WASI	25s
Tier 1 (windows-2022, stable)	48s
Web	1m 4s
Tier 1 (windows-2022, 1.63)	53s
Web with Atomics	1m 31s
Windows (stable-x86_64-gnu)	46s
Tier 1 (macos-14, stable)	20s
Windows (stable-i686-gnu)	37s
Windows (stable-i686-msvc)	42s
	22m 34s

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