build: Set shared linker flags in toolchain file #31395

These are required when cross-compiling shared libraries such as the kernel library.

This was discovered after attempting to cross-compile the kernel library and running into the following error:

[100%] Linking CXX shared library libbitcoinkernel.dylib
/usr/bin/ld: unrecognised emulation mode: llvm
Supported emulations: elf_x86_64 elf32_x86_64 elf_i386 elf_iamcu i386pep i386pe
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[3]: *** [src/kernel/CMakeFiles/bitcoinkernel.dir/build.make:1209: src/kernel/libbitcoinkernel.dylib] Error 1
gmake[2]: *** [CMakeFiles/Makefile2:1175: src/kernel/CMakeFiles/bitcoinkernel.dir/all] Error 2
gmake[1]: *** [CMakeFiles/Makefile2:1182: src/kernel/CMakeFiles/bitcoinkernel.dir/rule] Error 2
gmake: *** [Makefile:569: bitcoinkernel] Error 2

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/31395.

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ACK a8e04704f930de168fdddbb1c040dd70fbbe8ff7, tested on Ubuntu 24.04:

$ make -C depends -j 16 HOST=arm64-apple-darwin NO_QT=1
$ cmake -B build --toolchain depends/arm64-apple-darwin/toolchain.cmake -DBUILD_SHARED_LIBS=ON -DBUILD_KERNEL_LIB=ON
$ cmake --build build -j 16 -t bitcoinkernel

UPD. It seems to make sense to configure all CI jobs with -DBUILD_UTIL_CHAINSTATE=ON -DBUILD_KERNEL_LIB=ON -DBUILD_SHARED_LIBS=ON.

ACK a8e04704f930de168fdddbb1c040dd70fbbe8ff7

On master, was able to reproduce a build error cross compiling for arm64-apple-darwin and x86_64-apple-darwin:

$ make -C depends HOST=arm64-apple-darwin -j11
$ cmake -B build -DBUILD_KERNEL_LIB=ON --toolchain depends/arm64-apple-darwin/toolchain.cmake
$ cmake --build build -j11
...
[ 38%] Linking CXX shared library libbitcoinkernel.dylib
/usr/bin/ld: unrecognised emulation mode: llvm
Supported emulations: elf_x86_64 elf32_x86_64 elf_i386 elf_iamcu i386pep i386pe
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[2]: *** [src/kernel/CMakeFiles/bitcoinkernel.dir/build.make:1209: src/kernel/libbitcoinkernel.dylib] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:1723: src/kernel/CMakeFiles/bitcoinkernel.dir/all] Error 2
gmake[1]: *** Waiting for unfinished jobs....
[ 38%] Linking CXX static library libbitcoin_common.a
[ 38%] Built target bitcoin_common
gmake: *** [Makefile:146: all] Error 2

$ make -C depends HOST=x86_64-apple-darwin -j11
$ cmake -B build -DBUILD_KERNEL_LIB=ON --toolchain depends/x86_64-apple-darwin/toolchain.cmake
$ cmake --build build -j11
...
[ 72%] Linking CXX shared library libbitcoinkernel.dylib
/usr/bin/ld: unrecognised emulation mode: llvm
Supported emulations: elf_x86_64 elf32_x86_64 elf_i386 elf_iamcu i386pep i386pe
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[2]: *** [src/kernel/CMakeFiles/bitcoinkernel.dir/build.make:1211: src/kernel/libbitcoinkernel.dylib] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:1838: src/kernel/CMakeFiles/bitcoinkernel.dir/all] Error 2
gmake: *** [Makefile:146: all] Error 2

Was not able to reproduce the error building (on master) for riscv64-linux-gnu, arm-linux-gnueabihf, aarch64-linux-gnu, or x86_64-w64-mingw32.

On the PR branch, was able to successfully cross compile for arm64-apple-darwin, x86_64-apple-darwin, riscv64-linux-gnu, arm-linux-gnueabihf, and aarch64-linux-gnu. (sanity checked with file build/src/kernel/libbitcoinkernel.{so/dylib} to see correct binary info).

Ubuntu 24.04 with gcc 13.2.0 and clang 18.1.3.

Post-merge utACK a8e04704f930de168fdddbb1c040dd70fbbe8ff7