Use __builtin_setjmp instead of sigsetjmp. (#3360)

* Use `__builtin_setjmp` instead of `sigsetjmp`.

Use [`__builtin_setjmp`] instead of `sigsetjmp`, as it is implemented in
the compiler, performed inline, and saves much less state. This speeds up
calls into wasm by about 8% on my machine.

[`__builtin_setjmp`]: https://gcc.gnu.org/onlinedocs/gcc/Nonlocal-Gotos.html

* Add a comment confirming that 5 really is the documented size.

* Add a comment about callee-saved state and __builtin_setjmp.

* On clang on aarch64, use sigsetjmp.

* Fix a stray `#endif`.

crates/runtime/src/helpers.c

@@ -2,18 +2,44 @@
 #include <stdint.h>
 #include <stdlib.h>
 
+#ifdef CFG_TARGET_OS_windows
+
+#define platform_setjmp(buf) setjmp(buf)
+#define platform_longjmp(buf, arg) longjmp(buf, arg)
+typedef jmp_buf platform_jmp_buf;
+
+#elif defined(__clang__) && defined(__aarch64__)
+
+// Clang on aarch64 doesn't support `__builtin_setjmp`, so use `sigsetjmp`
+// from libc.
+//
 // Note that `sigsetjmp` and `siglongjmp` are used here where possible to
 // explicitly pass a 0 argument to `sigsetjmp` that we don't need to preserve
 // the process signal mask. This should make this call a bit faster b/c it
 // doesn't need to touch the kernel signal handling routines.
-#ifdef CFG_TARGET_OS_windows
-#define platform_setjmp(buf) setjmp(buf)
-#define platform_longjmp(buf, arg) longjmp(buf, arg)
-#define platform_jmp_buf jmp_buf
-#else
 #define platform_setjmp(buf) sigsetjmp(buf, 0)
 #define platform_longjmp(buf, arg) siglongjmp(buf, arg)
-#define platform_jmp_buf sigjmp_buf
+typedef sigjmp_buf platform_jmp_buf;
+
+#else
+
+// GCC and Clang both provide `__builtin_setjmp`/`__builtin_longjmp`, which
+// differ from plain `setjmp` and `longjmp` in that they're implemented by
+// the compiler inline rather than in libc, and the compiler can avoid saving
+// and restoring most of the registers. See the [GCC docs] and [clang docs]
+// for more information.
+//
+// Per the caveat in the GCC docs, this assumes that the host compiler (which
+// may be compiling for a generic architecture family) knows about all the
+// register state that Cranelift (which may be specializing for the hardware at
+// runtime) is assuming is callee-saved.
+//
+// [GCC docs]: https://gcc.gnu.org/onlinedocs/gcc/Nonlocal-Gotos.html
+// [clang docs]: https://llvm.org/docs/ExceptionHandling.html#llvm-eh-sjlj-setjmp
+#define platform_setjmp(buf) __builtin_setjmp(buf)
+#define platform_longjmp(buf, arg) __builtin_longjmp(buf, arg)
+typedef void *platform_jmp_buf[5]; // this is the documented size; see the docs links for details.
+
 #endif
 
 int wasmtime_setjmp(