Always add LC_BUILD_VERSION for metadata object files

As of Xcode 15 Apple's linker has become a bit more strict about the
warnings it produces. One of those new warnings requires all valid
Mach-O object files in an archive to have a LC_BUILD_VERSION load
command:

```
ld: warning: no platform load command found in 'ARCHIVE[arm64][2106](lib.rmeta)', assuming: iOS-simulator
```

This was already being done for Mac Catalyst so this change expands this
logic to include it for all Apple platforms. I filed this behavior
change as FB12546320 and was told it was the new intentional behavior.

compiler/rustc_target/src/spec/mod.rs

@@ -61,6 +61,8 @@ mod aix_base;
 mod android_base;
 mod apple_base;
 pub use apple_base::deployment_target as current_apple_deployment_target;
+pub use apple_base::platform as current_apple_platform;
+pub use apple_base::sdk_version as current_apple_sdk_version;
 mod avr_gnu_base;
 pub use avr_gnu_base::ef_avr_arch;
 mod bpf_base;