`long double` alignment tests fail on riscv32

When updating my project from 0.72.1 to bindgen 0.73.2, I started seeing this error.

header.h:

struct foo
{
    long long x;
    long double y;
};

bindgen header.h -- --target=riscv32-unknown-none-elf produces:

/* automatically generated by rust-bindgen 0.73.2 */

#[repr(C)]
#[repr(align(16))]
#[derive(Debug, Copy, Clone)]
pub struct foo {
    pub x: ::std::os::raw::c_longlong,
    pub y: u128,
}
#[allow(clippy::unnecessary_operation, clippy::identity_op)]
const _: () = {
    ["Size of foo"][::std::mem::size_of::<foo>() - 32usize];
    ["Alignment of foo"][::std::mem::align_of::<foo>() - 16usize];
    ["Offset of field: foo::x"][::std::mem::offset_of!(foo, x) - 0usize];
    ["Offset of field: foo::y"][::std::mem::offset_of!(foo, y) - 16usize];
};

This does not compile because the offset of y in the rust struct is 8, but in the C struct it is 16:

error[E0080]: attempt to compute `8_usize - 16_usize`, which would overflow
  --> bindings.rs:15:33
   |
15 |     ["Offset of field: foo::y"][::core::mem::offset_of!(foo, y) - 16usize];
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `_` failed here

The issue is that the long double has an alignment of 16 in riscv32, but it is represented as a u128, which has an alignment of 8 on riscv32.

I bisected the problem and found that this commit is the first that breaks things: https://github.com/rust-lang/rust-bindgen/commit/64f0939766c792822bd03c6733972e7e1067724e

On the immediate ancestor (cf8faab), I see this result instead, with 8 bytes of padding in between x and y:

/* automatically generated by rust-bindgen 0.73.1 */

#[derive(PartialEq, Eq, Copy, Clone, Debug, Hash)]
#[repr(C, align(8))]
pub struct __BindgenOpaqueArray8<T>(pub T);
impl<T: Copy + Default, const N: usize> Default for __BindgenOpaqueArray8<[T; N]> {
    fn default() -> Self {
        Self([<T as Default>::default(); N])
    }
}
#[repr(C)]
#[repr(align(16))]
#[derive(Debug, Copy, Clone)]
pub struct foo {
    pub x: ::core::ffi::c_longlong,
    pub __bindgen_padding_0: __BindgenOpaqueArray8<[u8; 8usize]>,
    pub y: u128,
}
#[allow(clippy::unnecessary_operation, clippy::identity_op)]
const _: () = {
    ["Size of foo"][::core::mem::size_of::<foo>() - 32usize];
    ["Alignment of foo"][::core::mem::align_of::<foo>() - 16usize];
    ["Offset of field: foo::x"][::core::mem::offset_of!(foo, x) - 0usize];
    ["Offset of field: foo::y"][::core::mem::offset_of!(foo, y) - 16usize];
};

Start by reproducing the issue with header.h and bindgen header.h -- --target=riscv32-unknown-none-elf, then inspect the generated bindings.rs and the change introduced by commit 64f0939766c792822bd03c6733972e7e1067724. Compare it with the 0.73.1 output. Done means the generated struct places y at offset 16 on riscv32 and the compile-time layout checks pass.