Reference array of structs instead of first member during memcpy

Reference array of structs instead of first member during memcpy

aarch64-tdep.c defines the following macro:

#define MEM_ALLOC(MEMS, LENGTH, RECORD_BUF) \
        do  \
          { \
            unsigned int mem_len = LENGTH; \
            if (mem_len) \
              { \
                MEMS =  XNEWVEC (struct aarch64_mem_r, mem_len);  \
                memcpy(&MEMS->len, &RECORD_BUF[0], \
                       sizeof(struct aarch64_mem_r) * LENGTH); \
              } \
          } \
          while (0)

This is simlpy allocating a new array and copying it. However, for
the destination address, it is actually copying into the first member
of the first element of the array (`&MEMS->len"). This elicits a
warning with GCC 12:

../../binutils-gdb/gdb/aarch64-tdep.c: In function ‘int aarch64_process_record(gdbarch*, regcache*, CORE_ADDR)’:
../../binutils-gdb/gdb/aarch64-tdep.c:3711:23: error: writing 16 bytes into a region of size 8 [-Werror=stringop-overflow=]
 3711 |                 memcpy(&MEMS->len, &RECORD_BUF[0], \
      |                       ^
../../binutils-gdb/gdb/aarch64-tdep.c:4394:3: note: in expansion of macro ‘MEM_ALLOC’
 4394 |   MEM_ALLOC (aarch64_insn_r->aarch64_mems, aarch64_insn_r->mem_rec_count,
      |   ^~~~~~~~~
../../binutils-gdb/gdb/aarch64-tdep.c:3721:12: note: destination object ‘aarch64_mem_r::len’ of size 8
 3721 |   uint64_t len;    /* Record length.  */
      |            ^~~

The simple fix is to reference the array, `MEMS' as the destination of the copy.

Tested by rebuilding.

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