gdb/testsuite: use proper int size for gdb.dwarf2/symbol_needs_eval*.exp

gdb/testsuite: use proper int size for gdb.dwarf2/symbol_needs_eval*.exp

We recently realized that symbol_needs_eval_fail.exp and
symbol_needs_eval_timeout.exp invalidly dereference an int (4 bytes on
x86_64) by reading 8 bytes (the size of a pointer).

Here how it goes:

In gdb/testsuite/gdb.dwarf2/symbol_needs_eval.c a global variable is
defined:

    int exec_mask = 1;

and later both tests build some DWARF using the assembler doing:

    set exec_mask_var [gdb_target_symbol exec_mask]
    ...
        DW_TAG_variable {
          {DW_AT_name a}
          {DW_AT_type :$int_type_label}
          {DW_AT_location {
            DW_OP_addr $exec_mask_var
            DW_OP_deref
            ...
          }
        }

The definition of the DW_OP_deref (from Dwarf5 2.5.1.3 Stack Operations)
says that "The size of the data retrieved from the dereferenced address
is the size of an address on the target machine."

On x86_64, the size of an int is 4 while the size of an address is 8.
The result is that when evaluating this expression, the debugger reads
outside of the `a` variable.

Fix this by using `DW_OP_deref_size $int_size` instead.  To achieve
this, this patch adds the necessary steps so we can figure out what
`sizeof(int)` evaluates to for the current target.

While at it, also change the definition of the int type in the assembled
DWARF information so we use the actual target's size for an int instead
of the literal 4.

Tested on x86_64 Linux.

Approved-By: Tom Tromey <tom@tromey.com>