Declare main in global NS, without extern "C"

The C++ standard doesn't allow a linkage-specification for main,
https://eel.is/c++draft/basic.start.main#3.sentence-5:

"The main function shall not be declared with a linkage-specification
([dcl.link])."

Bug: http://b/379133546
Test: m android.hardware.automotive.can-service \
    android.hardware.macsec-service \
    canhalconfigurator-aidl
Change-Id: Ifbc36bc1e3e05e6a12153d371871f4168bbe4cf8