Upgrade to spidermonkey 116 (#21)

Update to spidermonkey 116, and fix some build script issues that were
causing us to use the wrong tools.

This has one new commit on the fastly/ff-116 branch, to handle a new use
of `pthread_self` that showed up for debug builds only. The
corresponding commit is:


bytecodealliance/gecko-dev@ff10c17

Author: elliottt

build-engine.sh

@@ -108,6 +108,8 @@ cd "$working_dir"
 MOZCONFIG="${mozconfig}" \
 MOZ_FETCHES_DIR=~/.mozbuild \
 CC=~/.mozbuild/clang/bin/clang \
+CXX=~/.mozbuild/clang/bin/clang++ \
+AR=~/.mozbuild/clang/bin/llvm-ar \
   python3 "${working_dir}/gecko-dev/mach" \
   --no-interactive \
     build

gecko-revision

@@ -1 +1 @@
-6e34b2fcf46f8921de48d1400a6614c88884a6bb
+ff10c17d2ab60f0643f1a947a02c399882b3f1b4