[AVR] Use LLVM bitcast instructions rather than pointercasts to avoid…

… address space mismatch problems

This was suggested by @eddyb on Zulip:
https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Better.20supporting.20Harvard.20architectures/near/202043069

Bitcasts do not suffer from the same address space casting issues.

src/librustc_codegen_llvm/builder.rs

@@ -744,7 +744,7 @@ impl BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
     }
 
     fn pointercast(&mut self, val: &'ll Value, dest_ty: &'ll Type) -> &'ll Value {
-        unsafe { llvm::LLVMBuildPointerCast(self.llbuilder, val, dest_ty, UNNAMED) }
+        self.bitcast(val, dest_ty)
     }
 
     /* Comparisons */

src/librustc_codegen_llvm/llvm/ffi.rs

@@ -1346,12 +1346,6 @@ extern "C" {
         DestTy: &'a Type,
         Name: *const c_char,
     ) -> &'a Value;
-    pub fn LLVMBuildPointerCast(
-        B: &Builder<'a>,
-        Val: &'a Value,
-        DestTy: &'a Type,
-        Name: *const c_char,
-    ) -> &'a Value;
     pub fn LLVMRustBuildIntCast(
         B: &Builder<'a>,
         Val: &'a Value,