Use integer literals for builtin target_pointer_width fields

Also change target_pointer_width to pointer_width.

Preparation for a subsequent type change of
target_pointer_width to an integer together with a rename
to pointer_width.

On its own, this commit breaks the build. I don't like making
build-breaking commits, but in this instance I believe that it
makes review easier, as the "real" changes of this PR can be
seen much more easily.

Result of running:

find compiler/rustc_target/src/spec/ -type f -exec sed -i -e 's/target_pointer_width: "\(.*\)"\..*,/pointer_width: \1,/g' {} \;

compiler/rustc_target/src/spec/i686_unknown_linux_gnu.rs

@@ -10,7 +10,7 @@ pub fn target() -> Target {
     Target {
         llvm_target: "i686-unknown-linux-gnu".to_string(),
         target_endian: "little".to_string(),
-        target_pointer_width: "32".to_string(),
+        pointer_width: 32,
         target_c_int_width: "32".to_string(),
         data_layout: "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-\
             f64:32:64-f80:32-n8:16:32-S128"