compiler: make rustc_target have less weird reexports

rustc_target has had a lot of weird reexports for various reasons, but
now we're at a point where we can actually start reducing their number.
We remove weird shadowing-dependent behavior and import directly from
rustc_abi instead of doing weird renaming imports.

This is only incremental progress and does not entirely fix the crate.

compiler/rustc_target/src/asm/mod.rs

@@ -1,11 +1,11 @@
 use std::fmt;
 use std::str::FromStr;
 
+use rustc_abi::Size;
 use rustc_data_structures::fx::{FxHashMap, FxIndexSet};
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 use rustc_span::Symbol;
 
-use crate::abi::Size;
 use crate::spec::{RelocModel, Target};
 
 pub struct ModifierInfo {