ci: only build 64-bit version

For some time, Ubuntu has been phasing out support for 32-bit packages
(e.g., [here]). `ittapi` still builds a 32-bit version of the library,
which requires `gcc-multilib` to be installed (it provides
`/usr/include/bits/libc-header-start.h` when compiling with `-m32`). But
some GitHub inconsistency is getting in the way: `gcc-multilib` fails to
install on the GitHub runner though it can be installed locally.

[here]: https://canonical.com/blog/statement-on-32-bit-i386-packages-for-ubuntu-19-10-and-20-04-lts

While we figure out the 32-bit support story for `ittapi`, this change
disables the 32-bit build by forcing it to be 64-bit exclusively. Once
we decide whether to (a) remove 32-bit support or (b) fix the GitHub
runners to allow 32-bit builds, we can remove the TODO.

.github/workflows/main.yml

@@ -21,11 +21,10 @@ jobs:
     - uses: actions/checkout@v2
       with:
         submodules: recursive
-    - name: Install prerequisites
-      if: runner.os == 'Linux'
-      run: sudo apt-get install libc6-dev
     - name: Build C library
-      run: python buildall.py
+      # TODO: Ubuntu is phasing out support for 32-bit packages (e.g., `apt install gcc-multilib`
+      # fails on GitHub's runner); only build the 64-bit version for now.
+      run: python buildall.py --force_bits 64
 
   rust_format:
     name: Check Rust formatting