kernel: strip debug info in non-Debug builds

ReleaseSafe kept full DWARF/symtab because nothing set strip on the
kernel module, bloating the flashable image to 2.4MB (~1.4MB of which
was debug info). Strip for every optimize mode except Debug, so
release builds shrink to ~900KB while debug builds keep symbols.

build.zig

@@ -191,6 +191,7 @@ fn addKernel(
             .sanitize_c = .off, // the UBSan runtime needs f128/SSE support we don't provide
             .stack_check = false, // stack-probe calls have no runtime to land in
             .stack_protector = false,
+            .strip = optimize != .Debug, // DWARF info doubles the flashable image; keep it only for debug builds
             .imports = &.{
                 .{ .name = "boot-handoff", .module = modules.boot_handoff },
                 .{ .name = "abi", .module = modules.abi },