plugins: new syscalls plugin

plugins: new syscalls plugin

This commit adds a new syscalls plugin that displays the syscalls
as they are executed and returned. This plugin outputs the number
of the syscall as well as the syscall return value.

Works in *-user only.

Essentially, this commit restores:

  https://lists.gnu.org/archive/html/qemu-devel/2018-06/msg00846.html

by using the new QEMU plugin API.

Signed-off-by: Matthias Weckbecker <matthias@weckbecker.name>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200812115816.4454-1-matthias@weckbecker.name>
Message-Id: <20210312172821.31647-2-alex.bennee@linaro.org>