[clang][RISCV] Fix ABI mismatch between GCC and Clang (extension of integers on stack)

See <https://github.com/llvm/llvm-project/issues/57261> for full
details. Essentially, a previous version of the psABI indicated (by my
reading) that integer scalars passed on the stack were anyext. A [later
commit](https://github.com/riscv-non-isa/riscv-elf-psabi-doc/commit/cec39a064ee0e5b0129973fffab7e3ad1710498f)
changed this to indicate that they are in fact signext/zeroext just as
if they were passed in registers.

This patch adds the change in the release notes but doesn't add a flag
to retain the old behaviour. The hope is that it's sufficiently hard to
trigger an issue due to this that it isn't worthwhile doing so.

Differential Revision: https://reviews.llvm.org/D140401