core: cleanup sysgpu feature flag logic

* `@import("mach").core.gpu` has been renamed to `@import("mach").gpu`
* `pub const SYSGPUInterface` now has a default value (i.e. you do not need to write it in your main.zig, if you were.)
* You can now check `if (comptime mach.use_sysgpu)` for any conditional code you might have that should only run with sysgpu.

This (old):

```
pub const mach_core_options = core.ComptimeOptions{
    .use_wgpu = false,
    .use_sysgpu = true,
};
```

Has been replaced by this:

```
pub const use_sysgpu = true;
```

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

src/core/platform/common.zig

@@ -3,7 +3,7 @@ const builtin = @import("builtin");
 const mach = @import("../../main.zig");
 const gamemode = mach.gamemode;
 const mach_core = mach.core;
-const gpu = mach_core.gpu;
+const gpu = mach.gpu;
 
 pub inline fn printUnhandledErrorCallback(_: void, ty: gpu.ErrorType, message: [*:0]const u8) void {
     switch (ty) {