Treat LLVM libc's dynamic loader as the GNU ABI, which LLVM libc keeps, rather
than falling back to musl.

--- a/lib/std/zig/system.zig
+++ b/lib/std/zig/system.zig
@@ -599,6 +599,10 @@
                         if (look_for_ld) result.dynamic_linker = interp;
                         break;
                     }
+                } else if (std.mem.startsWith(u8, fs.path.basename(interp.get().?), "ld-llvm-libc")) {
+                    // LLVM libc keeps the glibc ABI
+                    result.abi = .gnu;
+                    if (look_for_ld) result.dynamic_linker = interp;
                 }
             },
             // We only need this for detecting glibc version.