wasilibc: 19 -> 20

pkgsCross.wasi32.llvmPackages: 12 -> 16

* It appears BULK_MEMORY_SOURCES no longer needs to be set to be empty
  to compile firefox. Without it, the build of wasilibc would fail if
  enableThreads is true.

* Include preliminary support for the experimental threads support in
  wasilibc which provides pthreads API. If wasilibc is built with
  support, is exposed via passthru and libcxx / libcxxabi are built with
  threads support accordingly.

  See also:
  - WebAssembly/wasi-sdk#274
  - WebAssembly/wasi-sdk#301
  - WebAssembly/wasi-sdk#314

  wasi-sdk ships it by default, but as a separate variant of libc which
  would be a hassle for us. Let's just make it optional for now.

  You can try it out using the following overlay:

      self: super: {
        wasilibc = super.wasilibc.override { enableThreads = true; };
      }

  Flags for libc++abi are copied from wasi-sdk.

pkgs/development/libraries/wasilibc/default.nix

@@ -2,6 +2,9 @@
 , buildPackages
 , fetchFromGitHub
 , lib
+# Enable experimental wasilibc pthread support
+, enableThreads ? false
+# For tests
 , firefox-unwrapped
 , firefox-esr-unwrapped
 }:
@@ -38,8 +41,7 @@ stdenv.mkDerivation {
  "SYSROOT_LIB:=$SYSROOT_LIB"
  "SYSROOT_INC:=$SYSROOT_INC"
  "SYSROOT_SHARE:=$SYSROOT_SHARE"
- # https://bugzilla.mozilla.org/show_bug.cgi?id=1773200
- "BULK_MEMORY_SOURCES:="
+ "THREAD_MODEL=${if enableThreads then "posix" else "single"}"
  )
 
  '';
@@ -53,8 +55,11 @@ stdenv.mkDerivation {
  ln -s $share/share/undefined-symbols.txt $out/lib/wasi.imports
  '';
 
- passthru.tests = {
- inherit firefox-unwrapped firefox-esr-unwrapped;
+ passthru = {
+ tests = {
+ inherit firefox-unwrapped firefox-esr-unwrapped;
+ };
+ hasThreads = enableThreads;
  };
 
  meta = with lib; {