Use gdb::array_view for value_array

This changes value_array to accept an array view.  I also replaced an
alloca with a std::vector in array_operation::evaluate.  This function
can work on any size of array, so it seems bad to use alloca.

Reviewed-by: John Baldwin <jhb@FreeBSD.org>
Approved-By: Simon Marchi <simon.marchi@efficios.com>