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.

.circleci/config.yml

@@ -22,12 +22,12 @@ commands:
     steps:
       - restore_cache:
           keys:
-            - llvm-source-12-v1
+            - llvm-source-13-v1
       - run:
           name: "Fetch LLVM source"
           command: make llvm-source
       - save_cache:
-          key: llvm-source-12-v1
+          key: llvm-source-13-v1
           paths:
             - llvm-project/clang/lib/Headers
             - llvm-project/clang/include
@@ -118,12 +118,12 @@ commands:
             - go-cache-macos-v3-{{ checksum "go.mod" }}
       - restore_cache:
           keys:
-            - llvm-source-12-macos-v1
+            - llvm-source-13-macos-v1
       - run:
           name: "Fetch LLVM source"
           command: make llvm-source
       - save_cache:
-          key: llvm-source-12-macos-v1
+          key: llvm-source-13-macos-v1
           paths:
             - llvm-project/clang/lib/Headers
             - llvm-project/clang/include
@@ -131,7 +131,7 @@ commands:
             - llvm-project/llvm/include
       - restore_cache:
           keys:
-            - llvm-build-12-macos-v1
+            - llvm-build-13-macos-v1
       - run:
           name: "Build LLVM"
           command: |
@@ -145,7 +145,7 @@ commands:
               find llvm-build -name CMakeFiles -prune -exec rm -r '{}' \;
             fi
       - save_cache:
-          key: llvm-build-12-macos-v1
+          key: llvm-build-13-macos-v1
           paths:
             llvm-build
       - restore_cache: