Set the vector registers type's name

Set the vector registers type's name

The "maintenance print register-groups" command reported the vector
registers type did not have a name, so set the name when we create it.

Before:

(gdb) maintenance print register-groups
 Name         Nr  Rel Offset    Size  Type            Groups
 v0            0    0      0     256 *1              all,general,vector
 ...
*1: Register type's name NULL.

After:

(gdb) maintenance print register-groups
 Name         Nr  Rel Offset    Size  Type            Groups
 v0            0    0      0     256 int32_t[64]     all,general,vector
...

Change-Id: I17fc4ddad488186f44f166dfd3577b33d80e98ad