[clang] [RISCV] Set __GCC_CONSTRUCTIVE_SIZE/__GCC_DESTRUCTIVE_SIZE to 64 for riscv64 (PR #162986)

[Sam Elliott via cfe-commits]

lenary wrote:

I added some RISC-V specific documentation here: https://github.com/riscv-non-isa/riscv-c-api-doc/pull/129

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