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/mipsisa64r6_unknown_linux_gnuabi64.rs

@@ -8,6 +8,7 @@ pub fn target() -> Target {
         data_layout: "E-m:e-i8:8:32-i16:16:32-i64:64-n32:64-S128".to_string(),
         arch: "mips64".to_string(),
         options: TargetOptions {
+            abi: "abi64".to_string(),
             endian: Endian::Big,
             // NOTE(mips64r6) matches C toolchain
             cpu: "mips64r6".to_string(),