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

[Philip Reames via cfe-commits]

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@@ -580,6 +580,8 @@ RISC-V Support
 - Add `-march=unset` to clear any previous `-march=` value. This ISA string will
   be computed from `-mcpu` or the platform default.
 
+- `__GCC_CONSTRUCTIVE_SIZE` and `__GCC_DESTRUCTIVE_SIZE` are changed to 64.
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preames wrote:

Can you add a link to some of the existing documentation on these being unstable?  Don't repeat the warning, but a pointer on where to look in docs would be good.  

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