Disable "-Wl,-plugin-opt,-import-instr-limit=5" on MacOS

This option is unsupported on the MacOS ld used in Android, leading to
compilation errors when LTO is enabled for binaries.

In particular the error includes:
```
ld64.lld: error: unknown argument '-plugin-opt'
ld64.lld: error: unknown argument '-import-instr-limit=5'
```

Test: m libc++ on a Mac OS machine
Bug: 288983988
Change-Id: I1873e0a77777832f833f211beb42633d462f04d5