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/wasm32_unknown_emscripten.rs

@@ -31,7 +31,7 @@ pub fn target() -> Target {
     Target {
         llvm_target: "wasm32-unknown-emscripten".to_string(),
         target_endian: "little".to_string(),
-        target_pointer_width: "32".to_string(),
+        pointer_width: 32,
         target_c_int_width: "32".to_string(),
         target_os: "emscripten".to_string(),
         target_env: String::new(),