cmac.c: optimize make_kn and move zero_iv to const segment.

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Backport https://github.com/openssl/openssl/commit/03cf7e784caa4c61febbf249be63cbae3e368ac9 to fix the following k1 stringop-overflow:

In function 'make_kn',
    inlined from 'make_kn' at crypto/cmac/cmac.c:81:13,
    inlined from 'CMAC_Init' at crypto/cmac/cmac.c:205:9:
crypto/cmac/cmac.c:92:20: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
   92 |         k1[bl - 1] ^= bl == 16 ? 0x87 : 0x1b;
      |         ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
crypto/cmac/cmac.c: In function 'CMAC_Init':
crypto/cmac/cmac.c:69:19: note: at offset [-2147483649, -1] into destination object 'k1' of size 32
   69 |     unsigned char k1[EVP_MAX_BLOCK_LENGTH];
      |                   ^~

Fixes:

• http://autobuild.buildroot.org/results/97b6333cdc7bad24aba7af1b04890679e0058299