[clang] [RISCV] Set __GCC_CONSTRUCTIVE_SIZE/__GCC_DESTRUCTIVE_SIZE to 64 (PR #162986)

[Philip Reames via cfe-commits]

https://github.com/preames approved this pull request.

LGTM w/minor comment addressed.

Maybe wait a day in case @topperc has concerns?

https://github.com/llvm/llvm-project/pull/162986