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.

targets/cortex-m0plus.json

@@ -2,5 +2,5 @@
 	"inherits": ["cortex-m"],
 	"llvm-target": "thumbv6m-unknown-unknown-eabi",
 	"cpu": "cortex-m0plus",
-	"features": "+armv6-m,+strict-align,+thumb-mode,-aes,-bf16,-cdecp0,-cdecp1,-cdecp2,-cdecp3,-cdecp4,-cdecp5,-cdecp6,-cdecp7,-crc,-crypto,-dotprod,-dsp,-fp16fml,-fullfp16,-hwdiv,-hwdiv-arm,-i8mm,-lob,-mve,-mve.fp,-ras,-sb,-sha2"
+	"features": "+armv6-m,+soft-float,+strict-align,+thumb-mode,-aes,-bf16,-cdecp0,-cdecp1,-cdecp2,-cdecp3,-cdecp4,-cdecp5,-cdecp6,-cdecp7,-crc,-crypto,-d32,-dotprod,-dsp,-fp-armv8,-fp-armv8d16,-fp-armv8d16sp,-fp-armv8sp,-fp16,-fp16fml,-fp64,-fpregs,-fullfp16,-hwdiv,-hwdiv-arm,-i8mm,-lob,-mve,-mve.fp,-neon,-ras,-sb,-sha2,-vfp2,-vfp2sp,-vfp3,-vfp3d16,-vfp3d16sp,-vfp3sp,-vfp4,-vfp4d16,-vfp4d16sp,-vfp4sp"
 }