Reduce startup size of PL_strtab

Perl 5.26 was using 512 as the default HvMAX for PL_strtab
this was increased in 528 to 2048, but we can start with
a lower value.

Note, that compiled programs using B::C would use one
accurate value from startup.

Changelog:

Author: atoomic

perl.c

@@ -353,7 +353,7 @@ perl_construct(pTHXx)
         /* SHAREKEYS tells us that the hash has its keys shared with PL_strtab,
          * which is not the case with PL_strtab itself */
         HvSHAREKEYS_off(PL_strtab);			/* mandatory */
-        hv_ksplit(PL_strtab, 1 << 11);
+        hv_ksplit(PL_strtab, 1 << 9); /* 5.26: 1 << 9 = 512 ; 5.28: 1 << 11 = 2048 */
     }
 
     Zero(PL_sv_consts, SV_CONSTS_COUNT, SV*);