- migrate more `-O3` to `${libc_opt_high_flag}`
- workaround a issue with `LLP64` in test. The overflow testing is
guarded by a constexpr but the literal overflow itself will still
trigger warnings.
Notice that for math smoke test, for some reasons, the
`${libc_opt_high_flag}` will be passed into `lld-link` which confuses
the linker so there are still some warnings leftover there. I can
investigate more when I have time.
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