Convert MAXIMUM_ZST_CAPACITY to be calculated in a

const instead of multiple target_pointer_width checks.

Author: moonheart08

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>() << 3) - 1); // Largest possible power of two
 
 /// A double-ended queue implemented with a growable ring buffer.
 ///