Fix singular Z2m ring creation and polynomial comparison

[user202729]

See the new test, previously it fails.

The n_Z2m ring creation is caused by #39075.

Singular source code contains

      // is exponent <=2^(8*sizeof(unsigned long))
      mp_bitcnt_t l=mpz_scan1 (modBase,0);
      if ((l>0) && (l<=8*sizeof(unsigned long)))
      {

so the limit is taken from there.

The polynomial comparison fix is workaround for Singular/Singular#1293. #39018 touches this part of the code, although I think it doesn't introduce the bug.

Fix #40838

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