Update targets to use target_abi

All eabi targets have target_abi = "eabi".
All eabihf targets have target_abi = "eabihf".
armv6_unknown_freebsd and armv7_unknown_freebsd have target_abi = "eabihf".
All abi64 targets have target_abi = "abi64".
All ilp32 targets have target_abi = "ilp32".
All softfloat targets have target_abi = "softfloat".
All *-uwp-windows-* targets have target_abi = "uwp".
All spe targets have target_abi = "spe".
All macabi targets have target_abi = "macabi".
aarch64-apple-ios-sim has target_abi = "sim".
x86_64-fortanix-unknown-sgx has target_abi = "fortanix".
x86_64-unknown-linux-gnux32 has target_abi = "x32".

Add FIXME entries for targets for which existing values need to change
once cfg_target_abi becomes stable. (All of them are tier 3 targets.)

Add a test for target_abi in `--print cfg`.

compiler/rustc_target/src/spec/powerpc_unknown_linux_gnuspe.rs

@@ -11,6 +11,11 @@ pub fn target() -> Target {
         pointer_width: 32,
         data_layout: "E-m:e-p:32:32-i64:64-n32".to_string(),
         arch: "powerpc".to_string(),
-        options: TargetOptions { endian: Endian::Big, mcount: "_mcount".to_string(), ..base },
+        options: TargetOptions {
+            abi: "spe".to_string(),
+            endian: Endian::Big,
+            mcount: "_mcount".to_string(),
+            ..base
+        },
     }
 }