* Constify string literals

Use -Wwrite-strings to force string literals to be of
type "const char[]", then fix up all warning fallout.

* Constify common infrastructure

Step one in allowing backend data to be readonly.
Minimal changes to backends for now; just set all pointers
in common structs that aren't modified to const.

* Constify AArch64 backend

Section size changes within libcapstone.so are

-.rodata               602587
-.data.rel.ro          228416
-.data                1003746
+.rodata               769051
+.data.rel.ro          241120
+.data                 824578

* Constify ARM backend

Section size changes within libcapstone.so are

-.rodata               769051
-.data.rel.ro          241120
-.data                 824578
+.rodata               959835
+.data.rel.ro          245120
+.data                 629506

* Constify Mips backend

Section size changes within libcapstone.so are

-.rodata               959835
-.data.rel.ro          245120
-.data                 629506
+.rodata              1069851
+.data.rel.ro          256416
+.data                 508194

* Constify PowerPC backend

Section size changes within libcapstone.so are

-.rodata              1069851
-.data.rel.ro          256416
-.data                 508194
+.rodata              1142715
+.data.rel.ro          272224
+.data                 419490

* Constify Sparc backend

Section size changes within libcapstone.so are

-.rodata              1142715
-.data.rel.ro          272224
-.data                 419490
+.rodata              1175227
+.data.rel.ro          277536
+.data                 381666

* Constify SystemZ backend

Section size changes within libcapstone.so are

-.rodata              1175227
-.data.rel.ro          277536
-.data                 381666
+.rodata              1221883
+.data.rel.ro          278016
+.data                 334498

* Constify X86 backend

Section size changes within libcapstone.so are

-.rodata              1221883
-.data.rel.ro          278016
-.data                 334498
+.rodata              1533531
+.data.rel.ro          281184
+.data                  19714

* Constify XCore backend

Section size changes within libcapstone.so are

-.rodata              1533531
-.data.rel.ro          281184
-.data                  19714
+.rodata              1553026
+.data.rel.ro          281280
+.data                     40