Update all scripts and build system to call python3, given python2's
upcoming doom. Use sys.maxsize instead of sys.maxint in one script
(maxint does not exist in python3).
If `seL4_arch_invocations` is an empty list, this will cause `nSeL4ArchInvocationLabels`
to be declared as 0, rather than `nInvocationLabels` as intented. This change correctly
respects the behaviour of the 'first' variable, and does an explicit assignment to
`nSeL4ArchInvocationLabels` should it be the first (and only) member of the enum.
Closes#53
If invocations are hidden behind config guards the count of how
many invocations there are will be wrong. The C compiler will
correctly assign the value with enum auto numbering, so there's
no need to override that here
Placing a config condition on the first define
in sel4arch.xml will fail as currently the first define
must start at the last define + 1 of the previous
invocation enum.
If this is ever required, autoconf.h will need to be passed
to the script so it can check config variables itself.
