gen_compile_commands: extract compiler command from a series of commands

The current gen_compile_commands.py assumes that objects are always
built by a single command.

It makes sense to support cases where objects are built by a series of
commands:

  cmd_<object> := <command1> ; <command2>

One use-case is that <command1> is a compiler command, and <command2>
an objtool command.

It allows *.cmd files to contain an objtool command so that any change
in it triggers object rebuilds.

If ; appears after the C source file, take the first command.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>