Implement support for providing information to Linux's perf JIT interface.

Implement support for providing information to Linux's perf JIT interface.

For more information on what this is, see:
 https://github.com/torvalds/linux/blob/master/tools/perf/Documentation/jit-interface.txt

[ChangeLog][QtQml][Profiling] QtQml can now write additional information so that
perf is able to give function names of JavaScript methods when profiling. To
enable this, set the environment variable QV4_PROFILE_WRITE_PERF_MAP=1 when
running the process in question.

Change-Id: I187c9b0792f40d93c89a986c0edb3c4487095cb7
Reviewed-by: Ulf Hermann <ulf.hermann@theqtcompany.com>