Workaround GCC `__ctzdi2` intrinsic linker errors

On Linux/i386 the 64bit `__builtin_ctzll()` instrinsic doesn't get
inlined by GCC but rather a short `__ctzdi2` runtime function is
inserted when needed into compiled object files.

This causes failures for the four test-cases

  TEST="T8639_api T8628 dynCompileExpr T5313"

with error messages of the kind

  dynCompileExpr: .../libraries/ghc-prim/dist-install/build/libHSghcpr_BE58KUgBe9ELCsPXiJ1Q2r.a: unknown symbol `__ctzdi2'
  dynCompileExpr: dynCompileExpr: unable to load package `ghc-prim'

This workaround forces GCC on 32bit x86 to to express `hs_ctz64` in
terms of the 32bit `__builtin_ctz()` (this is no loss, as there's no
64bit BSF instruction on i686 anyway) and thus avoid the problematic
out-of-line runtime function.

Note: `__builtin_ctzll()` is used since
      e0c1767d0ea8d12e0a4badf43682a08784e379c6 (re #9340)