riscv: airAsm rewrite

with this rewrite we can call functions inside of
inline assembly, enabling us to use the default start.zig logic

all that's left is to implement lr/sc loops for atomically manipulating
1 and 2 byte values, after which we can use the segfault handler logic.

test/behavior/pointers.zig

@@ -342,7 +342,6 @@ test "array initialization types" {
 test "null terminated pointer" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_riscv64) return error.SkipZigTest;
 
     const S = struct {
         fn doTheTest() !void {