[gdb/testsuite] Fix sizeof test in gdb.rust/simple.exp

[gdb/testsuite] Fix sizeof test in gdb.rust/simple.exp

On x86_64-linux, with test-case gdb.rust/simple.exp I get:
...
(gdb) print sizeof(e)^M
$52 = 24^M
(gdb) PASS: $exp: print sizeof(e)
...
but on i686-linux I get instead:
...
(gdb) print sizeof(e)^M
$52 = 20^M
(gdb) FAIL: $exp: print sizeof(e)
...

The variable e for which we print the size:
...
    let e = MoreComplicated::Two(73);
...
has type MoreComplicated which is defined like this:
...
pub struct HiBob {
    pub field1: i32,
    field2: u64,
}
  ...
enum MoreComplicated {
    One,
    Two(i32),
    Three(HiBob),
    Four{this: bool, is: u8, a: char, struct_: u64, variant: u32},
}
...

The answer to the question what the size of the enum should be seems to be
non-trivial [1][2], but AFAICT it doesn't seem to be illegal that the size can
differ between different platforms.

Fix this by accepting both 20 and 24 as valid size.

Tested on x86_64-linux and i686-linux.

Approved-By: Tom Tromey <tom@tromey.com>
[1] https://doc.rust-lang.org/reference/types/enum.html
[2] https://doc.rust-lang.org/reference/type-layout.html#the-rust-representation