By default the RISC-V target doesn't have the atomics standard extension enabled. The first RUN line in clang/test/CodeGen/atomic_ops.c doesn't specify a target triple, which means that on RISC-V Linux hosts it will target RISC-V, but because we use clang cc1 we don't get the toolchain driver functionality to automatically turn on the extensions implied by the target triple (riscv64-linux includes atomics). This causes the test to fail on RISC-V hosts.
This patch changes the test to have RUN lines for two targets, one with native atomics and one without. To work around FileCheck limitations and more accurately match the output, some tests now have separate prefixes for the two cases.