[clang] [clang][Modules] Raise empty.modulemap expected size to <70KB to fix RISC-V failure (PR #123959)

[Alex Bradbury via cfe-commits]

asb wrote:

> It could be due to builtins, I believe we always serialize them, and that would be different between targets. Does RISC-V have a lot of them compared to other targets?

That's almost certainly it then, thanks! We have a _lot_ of vector builtins in particular.

https://github.com/llvm/llvm-project/pull/123959