Add symlink for system lld

[haampie]

When building Julia with USE_SYSTEM_LLD:=1, the build system does not create a symlink to system lld in the install prefix. In the build logs I see it attempts to install lld:

$ zgrep -A7 'Install `lld`' .spack/spack-build-out.txt.gz 
# Install `lld` into private_libexecdir
/tmp/src/contrib/install.sh 755 /tmp/src/usr/tools/lld /tmp/install/libexec/julia/
cp: cannot stat '/tmp/src/usr/tools/lld': No such file or directory
chmod: cannot access '/tmp/install/libexec/julia//lld': No such file or directory
# Install `dsymutil` into private_libexecdir/
/tmp/src/contrib/install.sh 755 /tmp/src/usr/tools/dsymutil /tmp/install/libexec/julia/
cp: cannot stat '/tmp/src/usr/tools/dsymutil': No such file or directory
chmod: cannot access '/tmp/install/libexec/julia//dsymutil': No such file or directory

It would be convenient if Julia would create a symlink to lld, since USE_SYSTEM_LLD:=1 does not imply that there's only one lld on the system nor that it's always in PATH.

Edit: I see now that this is the intended behavior. The 7z symlink is created, and it looks like 02574e3 should do the same for lld, but it's just not working?

$ tree $(spack location -i julia)/libexec
/home/harmen/spack/opt/spack/linux-ubuntu23.10-zen2/gcc-13.2.0/julia-1.9.3-btpvx3biyaah65dy46pvc4nlwhehjymp/libexec
└── julia
    └── 7z -> /home/harmen/spack/opt/spack/linux-ubuntu23.10-zen2/gcc-13.2.0/p7zip-17.05-q6deo5fpxy7xncxu62ru4x5ypxhfdgl2/bin/7z

2 directories, 1 file

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Also, minor: use which could be potentially be replaced with shell builtin command -v to drop a dependency?