MDEV-19235 MariaDB Server compiled for 128 Indexes crashes at startup

With MAX_INDEXIES=64(default), key_map=Bitmap<64> is just a wrapper around
ulonglong and thus "trivial" (can be bzero-ed, or memcpy-ed, and stays
valid still)

With MAX_INDEXES=128, key_map = Bitmap<128> is not a "trivial" type
anymore. The implementation uses MY_BITMAP, and MY_BITMAP contains pointers
which make Bitmap invalid, when it is memcpy-ed/bzero-ed.

The problem in 10.4 is that there are many new key_map members, inside TABLE
or KEY, and those are often memcopied and bzeroed

The fix makes Bitmap "trivial", by inlining most of MY_BITMAP functionality.
pointers/heap allocations are not used anymore.

include/my_bit.h

@@ -122,6 +122,14 @@ static inline uint64 my_set_bits(int n)
   return (((1ULL << (n - 1)) - 1) << 1) | 1;
 }
 
+/* Create a mask of the significant bits for the last byte (1,3,7,..255) */
+static inline uchar last_byte_mask(uint bits)
+{
+  /* Get the number of used bits-1 (0..7) in the last byte */
+  unsigned int const used = (bits - 1U) & 7U;
+  /* Return bitmask for the significant bits */
+  return ((2U << used) - 1);
+}
 C_MODE_END
 
 #endif /* MY_BIT_INCLUDED */