[compiler-rt] Fix definition of `usize` on 32-bit Windows

32-bit Windows uses `unsigned int` for uintptr_t and size_t.
Commit 18e06e3e2f3d47433e1ed323b8725c76035fc1ac changed uptr to
unsigned long, so it no longer matches the real size_t/uintptr_t and
therefore the current definition of usize result in:
`error C2821: first formal parameter to 'operator new' must be 'size_t'`

However, the real problem is that uptr is wrong to work around the fact
that we have local SIZE_T and SSIZE_T typedefs that trample on the
basetsd.h definitions of the same name and therefore need to match
exactly. Unlike size_t/ssize_t the uppercase ones always use unsigned
long (even on 32-bit).

This commit works around the build breakage by keeping the existing
definitions of uptr/sptr and just changing usize. A follow-up change
will attempt to fix this properly.

Fixes: https://github.com/llvm/llvm-project/issues/101998

Reviewed By: mstorsjo

Pull Request: https://github.com/llvm/llvm-project/pull/106151