MDEV-15843 mysys: remove optimized memcpy from 18 years ago

While this code has remained dormant for 18 years, libc implementers
have used assembly features to gain improvements using achitecture
features optimized and by the buffer length like:
* https://svnweb.freebsd.org/base/head/lib/libc/amd64/string/memcmp.S
* https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/aarch64/memcmp.S
* https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/powerpc/powerpc64/memcpy.S

From an sysbench-1.0.6 oltp_read_only test on binary charset table:

x86_64:
was ptr_compare_0:
perf report -g --no-children:
+    3.37%  mysqld   mysqld               [.] hp_rec_hashnr
+    3.15%  mysqld   libc-2.26.so         [.] __memmove_avx_unaligned_erms
+    2.73%  mysqld   mysqld               [.] row_search_mvcc
+    1.97%  mysqld   mysqld               [.] rec_get_offsets_func
+    1.24%  mysqld   mysqld               [.] ptr_compare_0
+    1.14%  mysqld   mysqld               [.] my_qsort2

After: __memcmp_avx2_movbe
+    3.42%  mysqld   mysqld               [.] hp_rec_hashnr
+    2.96%  mysqld   libc-2.26.so         [.] __memmove_avx_unaligned_erms
+    2.91%  mysqld   mysqld               [.] row_search_mvcc
+    2.13%  mysqld   mysqld               [.] rec_get_offsets_func
+    1.18%  mysqld   libc-2.26.so         [.] __memcmp_avx2_movbe
+    1.04%  mysqld   mysqld               [.] evaluate_join_record
+    1.02%  mysqld   mysqld               [.] my_qsort2

Power9:
Before: ptr_compare_0
+    4.24%  mysqld   mysqld               [.] _Z15row_search_mvccPh15page_cur_mode_tP14row_prebuilt_tmm
+    2.18%  mysqld   mysqld               [.] hp_rec_hashnr
+    2.07%  mysqld   mysqld               [.] _Z20rec_get_offsets_funcPKhPK12dict_index_tPmbmPP16mem_block_info_t
+    1.60%  mysqld   mysqld               [.] _ZL20evaluate_join_recordP4JOINP13st_join_tablei
+    1.20%  mysqld   mysqld               [.] _ZN11ha_innobase13general_fetchEPhjj
+    1.05%  mysqld   mysqld               [.] _ZN17Item_func_between15val_int_cmp_intEv
+    0.92%  mysqld   mysqld               [.] _Z40row_sel_field_store_in_mysql_format_funcPhPK17mysql_row_templ_tPKhm
+    0.91%  mysqld   mysqld               [.] _ZNK10Item_param6PValue7val_intEPK19Type_std_attributes
+    0.84%  mysqld   mysqld               [.] ptr_compare_0

After: __memcmp_power8
+    2.29%  mysqld           mysqld                  [.] _Z15row_search_mvccPh15page_cur_mode_tP14row_prebuilt_tmm
+    1.32%  mysqld           mysqld                  [.] hp_rec_hashnr
+    1.18%  swapper          [kernel.kallsyms]       [k] power_enter_stop
+    1.12%  mysqld           mysqld                  [.] _Z20rec_get_offsets_funcPKhPK12dict_index_tPmbmPP16mem_block_info_t
+    0.87%  mysqld           mysqld                  [.] _ZL20evaluate_join_recordP4JOINP13st_join_tablei
+    0.87%  mysqld           [kernel.kallsyms]       [k] ___bpf_prog_run
+    0.76%  mysqld           libc-2.26.so            [.] __memcmp_power8
+    0.68%  mysqld           mysqld                  [.] _ZN11ha_innobase13general_fetchEPhjj
+    0.58%  mysqld           mysqld                  [.] _ZN17Item_func_between15val_int_cmp_intEv