cmake: Support setting the dynamic linker

[erikarvstedt]

This allows building Zig with the correct dynamic linker in environments where the auto detection mechanism fails, like in Nix dev shells.

This PR is a fixed version of #19631.

[erikarvstedt]

I've removed this newline because it made no sense semantically.

[erikarvstedt]

An alternative would be to add a generic ZIG_EXTRA_BUILD_ARGS variable that's appended to ZIG_BUILD_ARGS.
Actually, I'd prefer this, because it avoids duplicating Zig build options in cmake.

Usage:

# Set multiple build args
cmake -DZIG_EXTRA_BUILD_ARGS="-Ddynamic-linker=/my/linker;--libc;/my/libc/file;-Dstd-docs=true" 

[andrewrk]

Thanks!