gdb: fix ppc-sysv-tdep.c build on 32-bit platforms

The previous code triggered the following error on an i386 host:

/git/gdb/gdb/ppc-sysv-tdep.c:1764:34: error: non-constant-expression cannot be narrowed from type 'ULONGEST' (aka 'unsigned long long') to 'size_t' (aka 'unsigned int') in initializer list [-Wc++11-narrowing]
              unscaled.read ({writebuf, TYPE_LENGTH (valtype)},
                                        ^~~~~~~~~~~~~~~~~~~~~
/git/gdb/gdb/gdbtypes.h:2043:31: note: expanded from macro 'TYPE_LENGTH'
                              ^~~~~~~~~~~~~~~~~~
/git/gdb/gdb/ppc-sysv-tdep.c:1764:34: note: insert an explicit cast to silence this issue
              unscaled.read ({writebuf, TYPE_LENGTH (valtype)},
                                        ^~~~~~~~~~~~~~~~~~~~~
                                        static_cast<size_t>( )
/git/gdb/gdb/gdbtypes.h:2043:31: note: expanded from macro 'TYPE_LENGTH'
                              ^~~~~~~~~~~~~~~~~~
1 error generated.

Fix this by using gdb::make_array_view.