syscall_emul: style changes and FDArray refactor

Some members were defined as public when they should have been
privately declared so these were moved to the appropriate spot.

The operator[] had inline specified for for an in-class
definition which is redundant since inline definitions are
always implicitly inline.

Private members had the leading underscore applied to them to
denote that they're private (consistent with style guide).

Changed static const defined class variable into a constexpr
with brace-list initialization.

Change-Id: If3054416b57827d1542e9ebab428d67d0e767723
Reviewed-on: https://gem5-review.googlesource.com/12110
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

style: fix amd license and style issues

Change-Id: I26136fb49f743c4a597f8021cfd27f78897267b5
Reviewed-on: https://gem5-review.googlesource.com/10463
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>

syscall_emul: [PATCH 15/22] add clone/execve for threading and multiprocess simulations

Modifies the clone system call and adds execve system call. Requires allowing
processes to steal thread contexts from other processes in the same system
object and the ability to detach pieces of process state (such as MemState)
to allow dynamic sharing.

syscall_emul: [patch 10/22] refactor fdentry and add fdarray class

Several large changes happen in this patch.

The FDEntry class is rewritten so that file descriptors now correspond to
types: 'File' which is normal file-backed file with the file open on the
host machine, 'Pipe' which is a pipe that has been opened on the host machine,
and 'Device' which does not have an open file on the host yet acts as a pseudo
device with which to issue ioctls. Other types which might be added in the
future are directory entries and sockets (off the top of my head).

The FDArray class was create to hold most of the file descriptor handling
that was stuffed into the Process class. It uses shared pointers and
the std::array type to hold the FDEntries mentioned above.

The changes to these two classes needed to be propagated out to the rest
of the code so there were quite a few changes for that. Also, comments were
added where I thought they were needed to help others and extend our
DOxygen coverage.