rs6000: Add cast for int to enum conversion

rs6000: Add cast for int to enum conversion

Fixes this error:

/home/pedro/gdb/mygit/src/gdb/rs6000-tdep.c:3091:1: error: invalid conversion from ‘int’ to ‘bfd_architecture’ [-fpermissive]
 };
 ^
/home/pedro/gdb/mygit/src/gdb/rs6000-tdep.c: In function ‘void powerpc_set_vector_abi(char*, int, cmd_list_element*)’:
/home/pedro/gdb/mygit/src/gdb/rs6000-tdep.c:6063:28: error: invalid conversion from ‘int’ to ‘powerpc_vector_abi’ [-fpermissive]
  powerpc_vector_abi_global = vector_abi;
                            ^

gdb/ChangeLog:

	* rs6000-tdep.c (variants): Add (enum bfd_architecture) cast.