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.

cgo/libclang_config.go

@@ -1,5 +1,5 @@
 // +build !byollvm
-// +build !llvm12
+// +build !llvm12,!llvm13
 
 package cgo
 

cgo/libclang_config_llvm13.go

@@ -0,0 +1,14 @@
+// +build !byollvm
+// +build llvm13
+
+package cgo
+
+/*
+#cgo linux   CFLAGS:  -I/usr/lib/llvm-13/include
+#cgo darwin  CFLAGS:  -I/usr/local/opt/llvm@13/include
+#cgo freebsd CFLAGS:  -I/usr/local/llvm13/include
+#cgo linux   LDFLAGS: -L/usr/lib/llvm-13/lib -lclang
+#cgo darwin  LDFLAGS: -L/usr/local/opt/llvm@13/lib -lclang -lffi
+#cgo freebsd LDFLAGS: -L/usr/local/llvm13/lib -lclang
+*/
+import "C"