compiler: Switch default code model for loongarch64 to medium.

LLVM 21 will change the default, but we're making the change now to make
building Zig for loongarch64 less painful.

https://github.com/llvm/llvm-project/pull/132173

build.zig

@@ -668,24 +668,6 @@ fn addCompilerStep(b: *std.Build, options: AddCompilerStepOptions) *std.Build.St
         .strip = options.strip,
         .sanitize_thread = options.sanitize_thread,
         .single_threaded = options.single_threaded,
-        .code_model = switch (options.target.result.cpu.arch) {
-            // NB:
-            // For loongarch, LLVM supports only small, medium and large
-            // code model. If we don't explicitly specify the code model,
-            // the default value `small' will be used.
-            //
-            // Since zig binary itself is relatively large, using a `small'
-            // code model will cause
-            //
-            // relocation R_LARCH_B26 out of range
-            //
-            // error when linking a loongarch32/loongarch64 zig binary.
-            //
-            // Here we explicitly set code model to `medium' to avoid this
-            // error.
-            .loongarch32, .loongarch64 => .medium,
-            else => .default,
-        },
         .valgrind = options.valgrind,
     });