gd/hal: add getMsftOpcode to hci_hal_host

The getMsftOpcode function gets the MSFT opcode on Linux based
hosts. On non-Linux platforms, the returned 0 indicates MSFT
extension is not supported.

Bug: 246398494
Tag: #floss
Test: ./build.py --target test
Test: Manual test with next patch.

Change-Id: Ia6abf1ddaab7cd1b2f026323770d6fe8e1ab6a90