syscall_emul: style changes and FDArray refactor

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>