GNUmakefile: shrink TinyGo binaries on Linux

With these flags, the TinyGo binary gets 18.8MB (11.6%) smaller. That
seems like a quite useful win for such a small change!

This is only for Linux for now. MacOS and Windows can be tested later,
the flags for those probably need to be modified.

Originally inspired by:
https://discourse.llvm.org/t/state-of-the-art-for-reducing-executable-size-with-heavily-optimized-program/87952/18
There are some other flags like -Wl,--pack-dyn-relocs=relr that did not
shrink binary size in my testing, so I've left them out.

This also switches the linker to prefer mold or lld over the default
linker, since the system linker is usually ld.bfd which is very slow.
(Also, for some reason mold produces smaller binaries than lld).

.github/workflows/linux.yml

@@ -26,7 +26,7 @@ jobs:
         # tar: needed for actions/cache@v4
         # git+openssh: needed for checkout (I think?)
         # ruby: needed to install fpm
-        run: apk add tar git openssh make g++ ruby-dev
+        run: apk add tar git openssh make g++ ruby-dev mold
       - name: Work around CVE-2022-24765
         # We're not on a multi-user machine, so this is safe.
         run: git config --global --add safe.directory "$GITHUB_WORKSPACE"
@@ -73,7 +73,7 @@ jobs:
         uses: actions/cache/restore@v4
         id: cache-llvm-build
         with:
-          key: llvm-build-19-linux-alpine-v1
+          key: llvm-build-19-linux-alpine-v2
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'
@@ -222,7 +222,7 @@ jobs:
         uses: actions/cache/restore@v4
         id: cache-llvm-build
         with:
-          key: llvm-build-19-linux-asserts-v1
+          key: llvm-build-19-linux-asserts-v2
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'
@@ -329,7 +329,7 @@ jobs:
         uses: actions/cache/restore@v4
         id: cache-llvm-build
         with:
-          key: llvm-build-19-linux-${{ matrix.goarch }}-v3
+          key: llvm-build-19-linux-${{ matrix.goarch }}-v4
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'