Auto merge of #52787 - riscv-rust:riscv-rust-pr, r=alexcrichton

Enable RISCV

- Enable LLVM backend.
- Implement call abi.
- Add built-in target riscv32imac-unknown-none.
- Enable CI.

src/rustllvm/PassWrapper.cpp

@@ -171,6 +171,12 @@ bool LLVMRustPassManagerBuilderPopulateThinLTOPassManager(
 #define SUBTARGET_MSP430
 #endif
 
+#ifdef LLVM_COMPONENT_RISCV
+#define SUBTARGET_RISCV SUBTARGET(RISCV)
+#else
+#define SUBTARGET_RISCV
+#endif
+
 #ifdef LLVM_COMPONENT_SPARC
 #define SUBTARGET_SPARC SUBTARGET(Sparc)
 #else
@@ -192,7 +198,8 @@ bool LLVMRustPassManagerBuilderPopulateThinLTOPassManager(
   SUBTARGET_SYSTEMZ                                                            \
   SUBTARGET_MSP430                                                             \
   SUBTARGET_SPARC                                                              \
-  SUBTARGET_HEXAGON
+  SUBTARGET_HEXAGON                                                            \
+  SUBTARGET_RISCV                                                              \
 
 #define SUBTARGET(x)                                                           \
   namespace llvm {                                                             \