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.

.circleci/config.yml

@@ -101,12 +101,12 @@ jobs:
           # "make lint" fails before go 1.21 because internal/tools/go.mod specifies packages that require go 1.21
           fmt-check: false
     resource_class: large
-  test-llvm19-go124:
+  test-llvm20-go124:
     docker:
       - image: golang:1.24-bullseye
     steps:
       - test-linux:
-          llvm: "19"
+          llvm: "20"
     resource_class: large
 
 workflows:
@@ -115,5 +115,5 @@ workflows:
       # This tests our lowest supported versions of Go and LLVM, to make sure at
       # least the smoke tests still pass.
       - test-llvm15-go119
-      # This tests LLVM 19 support when linking against system libraries.
-      - test-llvm19-go124
+      # This tests LLVM 20 support when linking against system libraries.
+      - test-llvm20-go124