CI: streamline compilation commands

— add “-s” to LDFLAGS instead of manually calling strip
— on GNU/Linux, do not pass “-static” to LDFLAGS. We can dynamically link
  against glibc, since in Artifacts.toml we explicitly mention that we compile
  for a glibc system (and not musl)
— use --host=i686-linux-gnu to compile for GNU/Linux 32-bit, instead of passing “-m32”
  N.B.: Under Debian, g++-i686-linux-gnu must be installed. Otherwise this will
  silently use the 64-bit compiler
— for symmetry, use --host=x86_64-linux-gnu for GNU/Linux 64-bit
— on macOS, pass “-static-libgcc” through LDFLAGS rather than CXXFLAGS; this
  requires setting CC to gcc-10
— use better names for the jobs