gdb: fix value.c build on 32-bits

gdb: fix value.c build on 32-bits

When building on ARM (32-bits), we errors like this:

    /home/smarchi/src/binutils-gdb/gdb/value.c: In function 'gdb::array_view<const unsigned char> value_contents_for_printing(value*)':
    /home/smarchi/src/binutils-gdb/gdb/value.c:1252:35: error: narrowing conversion of 'length' from 'ULONGEST' {aka 'long long unsigned int'} to 'size_t' {aka 'unsigned int'} [-Werror=narrowing]
     1252 |   return {value->contents.get (), length};
          |                                   ^~~~~~

Fix that by using gdb::make_array_view, which does the appropriate
conversion.

Change-Id: I7d6f2e75d7440d248b8fb18f8272ee92954b404d