remove the ZIG_SYSTEM_LINKER_HACK feature

[andrewrk]

ZIG_SYSTEM_LINKER_HACK is an environment variable that Zig inspects on macOS when building natively, and will execute the system linker, ld64, as a child process, rather than using LLD or self-hosted linker.

zig/src/Compilation.zig

Line 764 in 2d11967

const system_linker_hack = std.os.getenv("ZIG_SYSTEM_LINKER_HACK") != null;

Don't get comfortable with it. This is a temporary workaround until our Mach-O linking code is sufficient to justify removing it.

Related: #8727

This issue blocks Zig 1.0.