Add a .bss-like scheme for encoded const allocs

compiler/rustc_abi/src/lib.rs

@@ -791,7 +791,7 @@ impl Align {
     }
 
     #[inline]
-    pub fn bytes(self) -> u64 {
+    pub const fn bytes(self) -> u64 {
         1 << self.pow2
     }
 
@@ -801,7 +801,7 @@ impl Align {
     }
 
     #[inline]
-    pub fn bits(self) -> u64 {
+    pub const fn bits(self) -> u64 {
         self.bytes() * 8
     }