[llvm] [RISCV] Add additional fence for amocas when required by recent ABI change (PR #101023)

[Patrick O'Neill via llvm-commits]

patrick-rivos wrote:

This PR passes the tests I wrote on the GCC side to confirm the compatibility fence is added/omitted when not needed.
[zacas-rvwmo-compare-exchange-compatability-mapping.cc](https://github.com/gcc-mirror/gcc/blob/master/gcc/testsuite/gcc.target/riscv/amo/zacas-rvwmo-compare-exchange-compatability-mapping.cc)
[zacas-rvwmo-compare-exchange-compatability-mapping-no-fence.c](https://github.com/gcc-mirror/gcc/blob/master/gcc/testsuite/gcc.target/riscv/amo/zacas-rvwmo-compare-exchange-compatability-mapping-no-fence.c)
[zacas-ztso-compare-exchange-compatability-mapping.cc](https://github.com/gcc-mirror/gcc/blob/master/gcc/testsuite/gcc.target/riscv/amo/zacas-ztso-compare-exchange-compatability-mapping.cc)
[zacas-ztso-compare-exchange-compatability-mapping-no-fence.c](https://github.com/gcc-mirror/gcc/blob/master/gcc/testsuite/gcc.target/riscv/amo/zacas-ztso-compare-exchange-compatability-mapping-no-fence.c)


https://github.com/llvm/llvm-project/pull/101023