Introduce codegen_iconst_u128 #1796

Job	Run time
abi_cafe (windows-latest, x86_64-pc-windows-msvc)	10m 10s
abi_cafe (ubuntu-latest, x86_64-unknown-linux-gnu)	7m 28s
abi_cafe (ubuntu-24.04-arm, aarch64-unknown-linux-gnu)	5m 37s
abi_cafe (macos-latest, aarch64-apple-darwin)	5m 3s
abi_cafe (macos-15-intel, x86_64-apple-darwin)	14m 8s
	42m 26s

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