rust: alloc: add ARCH_KMALLOC_MINALIGN to bindgen blocklist

rust: alloc: add ARCH_KMALLOC_MINALIGN to bindgen blocklist

For some architectures, such as X86_64, ARCH_KMALLOC_MINALIGN is not
resolvable for bindgen. E.g. due to being defined as
__alignof__(unsigned long long).

Hence, we have to create a rust helper, i.e. let the C compiler evaluate
the expression and store it in a const.

However, if for other architectures, such as arm64,
ARCH_KMALLOC_MINALIGN does evaluate to something that can be directly
processed by bindgen, we end up with multiple definitions of
ARCH_KMALLOC_MINALIGN in the generated bindings.

	error[E0428]: the name `ARCH_KMALLOC_MINALIGN` is defined multiple times
	      --> /builddir/build/BUILD/kernel-6.17.0-build/kernel-next-20250818/linux-6.17.0-0.0.next.20250818.423.vanilla.fc44.aarch64/rust/bindings/bindings_generated.rs:134545:1
	       |
	9622   | pub const ARCH_KMALLOC_MINALIGN: u32 = 8;
	       | ----------------------------------------- previous definition of the value `ARCH_KMALLOC_MINALIGN` here
	...
	134545 | pub const ARCH_KMALLOC_MINALIGN: usize = 8;
	       | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ARCH_KMALLOC_MINALIGN` redefined here
	       |
	       = note: `ARCH_KMALLOC_MINALIGN` must be defined only once in the value namespace of this module

To fix this up, add ARCH_KMALLOC_MINALIGN to the blocklist of bindgen,
such that we always only generate the symbol from the rust helper.

Reported-by: Thorsten Leemhuis <linux@leemhuis.info>
Closes: https://lore.kernel.org/all/8aa05f08-ef6e-4dfe-9453-beaab7b3cb98@leemhuis.info/
Fixes: 1b1a946dc2b5 ("rust: alloc: specify the minimum alignment of each allocator")
Tested-by: Thorsten Leemhuis <linux@leemhuis.info>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250818180923.192042-1-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>