Rollup merge of #76481 - moonheart08:vec_deque_constify, r=sfackler

Convert repetitive target_pointer_width checks to const solution.

Simply a quick code tidying change. Not sure if more needs to be said.

Author: tmandry

library/alloc/src/collections/vec_deque.rs

@@ -32,12 +32,8 @@ mod tests;
 
 const INITIAL_CAPACITY: usize = 7; // 2^3 - 1
 const MINIMUM_CAPACITY: usize = 1; // 2 - 1
-#[cfg(target_pointer_width = "16")]
-const MAXIMUM_ZST_CAPACITY: usize = 1 << (16 - 1); // Largest possible power of two
-#[cfg(target_pointer_width = "32")]
-const MAXIMUM_ZST_CAPACITY: usize = 1 << (32 - 1); // Largest possible power of two
-#[cfg(target_pointer_width = "64")]
-const MAXIMUM_ZST_CAPACITY: usize = 1 << (64 - 1); // Largest possible power of two
+
+const MAXIMUM_ZST_CAPACITY: usize = 1 << (core::mem::size_of::<usize>() * 8 - 1); // Largest possible power of two
 
 /// A double-ended queue implemented with a growable ring buffer.
 ///