Enable libunwind for rtems #1140

Job	Run time
Rustfmt	7s
iOS (aarch64-apple-ios, iphoneos)	18s
iOS (x86_64-apple-ios, iphonesimulator)	14s
Windows AArch64	54s
Miri	2m 34s
Docker (aarch64-unknown-linux-gnu)	51s
Build Targets (wasm32-unknown-unknown)	17s
Docker (arm-unknown-linux-gnueabihf)	49s
Build Targets (wasm32-wasi)	13s
MSRV (ubuntu-20.04)	18s
Docker (armv7-unknown-linux-gnueabihf)	40s
Build Targets (x86_64-unknown-fuchsia)	1s
MSRV (windows-latest)	29s
Docker (i586-unknown-linux-gnu)	29s
Build Targets (x86_64-fortanix-unknown-sgx)	19s
Docker (i686-unknown-linux-gnu)	32s
Build Targets (x86_64-unknown-illumos)	16s
Test (ubuntu-20.04, stable)	1m 45s
Docker (powerpc64-unknown-linux-gnu)	52s
Test (ubuntu-20.04, beta)	19s
Docker (s390x-unknown-linux-gnu)	38s
Test (ubuntu-20.04, nightly)	16s
Docker (x86_64-pc-windows-gnu)	46s
Test (ubuntu-24.04, stable)	1m 57s
Docker (x86_64-unknown-linux-gnu)	37s
Test (ubuntu-24.04, beta)	19s
Docker (x86_64-unknown-linux-musl)	33s
Test (ubuntu-24.04, nightly)	16s
Docker (arm-linux-androideabi)	1m 50s
Test (macos-latest, stable)	1m 2s
Docker (armv7-linux-androideabi)	1m 23s
Test (macos-latest, nightly)	21s
Docker (aarch64-linux-android)	1m 36s
Test (windows-latest, stable-x86_64-msvc)	2m 3s
Docker (i686-linux-android)	1m 31s
Test (windows-latest, stable-i686-msvc)	2m 13s
Docker (x86_64-linux-android)	1m 15s
Test (windows-latest, stable-x86_64-gnu)	2m 11s
Test (windows-latest, nightly-x86_64-msvc)	33s
Test (windows-latest, nightly-i686-msvc)	38s
Test (windows-latest, nightly-x86_64-gnu)	33s
	34m 48s

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