plugins/api: split out binary path/start/end/entry code

plugins/api: split out binary path/start/end/entry code

To move the main api.c to a single build compilation object we need to
start splitting out user and system specific code. As we need to grob
around host headers we move these particular helpers into the *-user
mode directories.

The binary/start/end/entry helpers are all NOPs for system mode.

While using the plugin-api.c.inc trick means we build for both
linux-user and bsd-user the BSD user-mode command line is still
missing -plugin. This can be enabled once we have reliable check-tcg
tests working for the BSDs.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250304222439.2035603-27-alex.bennee@linaro.org>