Add a new "dumpn" pseudo-opcode that dumps N copies of the same value
to the stage buffer. It's basically the same as "dump" but much faster
when dumping the same value a lot of times, for example, when writing
initial values for large arrays.
On my Linux system the time it took to dump a 100000000 cell array went
down from 20 to 8 seconds in Release configuration, i.e. 2.5 times faster!
I haven't profiled further yet (Visual Studio 2017 profiler is broken,
gprof won't output anything (I'm probably doing it wrong), I might try
valgrind later).
This adds a new function pc_createtmpsrc() that creates a temporary
source file for writing. Also, mkstemp() is now used not only on Mac
OS X but on Linux (and other Unixes) as well.
