add riscv64 backend for cranelift. (#4271)

Add a RISC-V 64 (`riscv64`, RV64GC) backend.

Co-authored-by: yuyang <756445638@qq.com>
Co-authored-by: Chris Fallin <chris@cfallin.org>
Co-authored-by: Afonso Bordado <afonsobordado@az8.co>

crates/runtime/src/traphandlers/backtrace/riscv64.rs

@@ -0,0 +1,21 @@
+//
+pub unsafe fn get_next_older_pc_from_fp(fp: usize) -> usize {
+    *(fp as *mut usize).offset(1)
+}
+
+// And the current frame pointer points to the next older frame pointer.
+pub const NEXT_OLDER_FP_FROM_FP_OFFSET: usize = 0;
+
+pub fn reached_entry_sp(fp: usize, first_wasm_sp: usize) -> bool {
+    // Calls in riscv64 push two i64s (old FP and return PC) so our entry SP is
+    // two i64s above the first Wasm FP.
+    fp == first_wasm_sp - 16
+}
+
+pub fn assert_entry_sp_is_aligned(sp: usize) {
+    assert_eq!(sp % 16, 0, "stack should always be aligned to 16");
+}
+
+pub fn assert_fp_is_aligned(fp: usize) {
+    assert_eq!(fp % 16, 0, "stack should always be aligned to 16");
+}