dlmalloc in 4GB mode attempts to sbrk() in more than 3GB.

Running without the special gotcha at https://github.com/emscripten-core/emscripten/pull/25126#discussion_r2316901481 , the test `browser.test_dlmalloc_3gb` will fail.

`sbrk()` spec is clear that the parameter is supposed to be signed, with the ability to decrement memory.

But that means that in a 32-bit build one cannot use sbrk() to allocate more than 2GB at a time (but must call it twice).

In https://github.com/emscripten-core/emscripten/pull/25126 I add a new API `sbrk64()` which takes a signed 64-bit parameter even in 32-bit modes, to avoid this flaw.

But dlmalloc will still need fixing to account for this.