all: add support for LLVM 20

This adds support for the 'go install' case, where we link against the
system LLVM (Homebrew, Linux distro version, etc). This does not have an
effect on `make`, which still uses LLVM 19 for now.

The main reason for doing this is because newer distros like Fedora 42
and Homebrew have switched to LLVM 20, so switching to this newer
version helps those people. (I'm one of those people, I'm on Fedora 42).

There are two small changes for WebAssembly included:

  * nontrapping-fptoint was enabled in the wasmbuiltin library used for
    wasm-unknown. This matches wasm-unknown which already had this
    enabled, so it doesn't add any new instructions.
  * bulk-memory was enabled in wasi-libc. This was previously enabled in
    all the other WebAssembly targets (wasm, wasip1, wasip2) so this
    does't add any new instructions. I think it was also enabled in the
    wasi-libc build before
    https://github.com/tinygo-org/tinygo/pull/4820 but I don't know for
    sure.

.github/workflows/build-macos.yml

@@ -117,7 +117,7 @@ jobs:
     runs-on: macos-latest
     strategy:
       matrix:
-        version: [16, 17, 18, 19]
+        version: [16, 17, 18, 19, 20]
     steps:
       - name: Set up Homebrew
         uses: Homebrew/actions/setup-homebrew@master
@@ -141,8 +141,8 @@ jobs:
       - name: Check binary
         run: tinygo version
       - name: Build TinyGo (default LLVM)
-        if: matrix.version == 19
+        if: matrix.version == 20
         run: go install
       - name: Check binary
-        if: matrix.version == 19
+        if: matrix.version == 20
         run: tinygo version

.github/workflows/sizediff-install-pkgs.sh

@@ -2,11 +2,11 @@
 # still works after checking out the dev branch (that is, when going from LLVM
 # 16 to LLVM 17 for example, both Clang 16 and Clang 17 are installed).
 
-echo 'deb https://apt.llvm.org/noble/ llvm-toolchain-noble-19 main' | sudo tee /etc/apt/sources.list.d/llvm.list
+echo 'deb https://apt.llvm.org/noble/ llvm-toolchain-noble-20 main' | sudo tee /etc/apt/sources.list.d/llvm.list
 wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
 sudo apt-get update
 sudo apt-get install --no-install-recommends -y \
-    llvm-19-dev \
-    clang-19 \
-    libclang-19-dev \
-    lld-19
+    llvm-20-dev \
+    clang-20 \
+    libclang-20-dev \
+    lld-20