all: switch to LLVM 13

This adds support for building with `-tags=llvm13` and switches to LLVM
13 for tinygo binaries that are statically linked against LLVM.

Some notes on this commit:

  * Added `-mfloat-abi=soft` to all Cortex-M targets because otherwise
    nrfx would complain that floating point was enabled on Cortex-M0.
    That's not the case, but with `-mfloat-abi=soft` the `__SOFTFP__`
    macro is defined which silences this warning.
    See: https://reviews.llvm.org/D100372
  * Changed from `--sysroot=<root>` to `-nostdlib -isystem <root>` for
    musl because with Clang 13, even with `--sysroot` some system
    libraries are used which we don't want.
  * Changed all `-Xclang -internal-isystem -Xclang` to simply
    `-isystem`, for consistency with the above change. It appears to
    have the same effect.
  * Moved WebAssembly function declarations to the top of the file in
    task_asyncify_wasm.S because (apparently) the assembler has become
    more strict.

src/internal/task/task_asyncify_wasm.S

@@ -1,5 +1,14 @@
 .globaltype __stack_pointer, i32
 
+.functype start_unwind (i32) -> ()
+.import_module start_unwind, asyncify
+.functype stop_unwind () -> ()
+.import_module stop_unwind, asyncify
+.functype start_rewind (i32) -> ()
+.import_module start_rewind, asyncify
+.functype stop_rewind () -> ()
+.import_module stop_rewind, asyncify
+
 .global  tinygo_unwind
 .hidden  tinygo_unwind
 .type    tinygo_unwind,@function
@@ -84,15 +93,6 @@ tinygo_rewind: // func (state *state) rewind()
     return
     end_function
 
-.functype start_unwind (i32) -> ()
-.import_module start_unwind, asyncify
-.functype stop_unwind () -> ()
-.import_module stop_unwind, asyncify
-.functype start_rewind (i32) -> ()
-.import_module start_rewind, asyncify
-.functype stop_rewind () -> ()
-.import_module stop_rewind, asyncify
-
         .hidden tinygo_rewinding                # @tinygo_rewinding
         .type   tinygo_rewinding,@object
         .section        .bss.tinygo_rewinding,"",@