Cleanup trampoline size checks. (mono/mono#15624)

Cleanup trampoline size checks. (mono/mono#15624)

Cleanup trampoline size checks.

- named `int` everywhere
- `assert` everywhere
- assert `<=` everywhere instead of `<`
- `const` almost everywhere
Names not consistent, following existing code (`size`, `buf_len`, `kMaxCode`.)

Only touching x86/amd64, granted.

Could be better -- make them two pass and avoid the presizing.

Commit migrated from https://github.com/mono/mono/commit/5390b796dd9c16eaded61c406a4cea9c66ea8905