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.

.github/workflows/linux.yml

@@ -38,7 +38,7 @@ jobs:
         uses: actions/cache@v2
         id: cache-llvm-source
         with:
-          key: llvm-source-12-linux-v1
+          key: llvm-source-13-linux-v1
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include
@@ -51,7 +51,7 @@ jobs:
         uses: actions/cache@v2
         id: cache-llvm-build
         with:
-          key: llvm-build-12-linux-v1
+          key: llvm-build-13-linux-v1
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'
@@ -174,7 +174,7 @@ jobs:
         uses: actions/cache@v2
         id: cache-llvm-source
         with:
-          key: llvm-source-12-linux-asserts-v1
+          key: llvm-source-13-linux-asserts-v1
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include
@@ -187,7 +187,7 @@ jobs:
         uses: actions/cache@v2
         id: cache-llvm-build
         with:
-          key: llvm-build-12-linux-asserts-v1
+          key: llvm-build-13-linux-asserts-v1
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'

.github/workflows/windows.yml

@@ -34,7 +34,7 @@ jobs:
         uses: actions/cache@v2
         id: cache-llvm-source
         with:
-          key: llvm-source-12-windows-v1
+          key: llvm-source-13-windows-v1
           path: |
             llvm-project/clang/lib/Headers
             llvm-project/clang/include
@@ -47,7 +47,7 @@ jobs:
         uses: actions/cache@v2
         id: cache-llvm-build
         with:
-          key: llvm-build-12-windows-v1
+          key: llvm-build-13-windows-v1
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'