Add Linux perf JIT support (/tmp/perf-$pid.map)

'perf' is the standard builtin tool for performance analysis on recent
Linux kernel. Its source code is shipped within the kernel repository.

'perf' has basic support for JIT. For each process, it can read a file
named /tmp/perf-$PID.map. This file contains mapping from address
range to function name in the format:

  41187e2a 1a JIT_PPC_804a33fc

with the following entries:

 1. beginning of the range (hexadecimal);
 2. size of the range (hexadecimal);
 3. name of the function.

We supply the PowerPC address of the basic block as function name.

Usage:

    DOLPHIN_PERF=1 dolphin-emu &
    perf record -F99 -p $(pgrep dolphin-emu) --cal-graph dwarf
    perf script | stackcollapse-perf.pl | grep JIT_PPC_ | flamegraph.pl > profile.svg

Issues:

  * perf does not have support for region invalidation. It reads the
    file in postprocessing. It probably does not work very well if a
    JIT region is reused for another basic block: wrong results should
    be expected in this case. Currently, nothing is done to prevent
    this.

To be fixed:

  * Add a GUI option.

Long term evolutions:

  * add the function name as well in the file;
  * add support for ARM JIT;
  * add support for DSP JIT;
  * stack unwinding support for JIT functions.