mesa/st/glsl_to_tgsi: Add method to collect some TGSI statistics

mesa/st/glsl_to_tgsi: Add method to collect some TGSI statistics

When mesa is compiled in debug mode then this adds the possibility
to print out some statistics about the translated and optimized TGSI
shaders to a file.

The functionality is enabled by setting the environment variable

   GLSL_TO_TGSI_PRINT_STATS

to the file name where the statistics should be collected. The file is
opened in append mode so that statistics from various runs will be
accumulated.

v4: Make accress to log file thread save (thanks for pointing this out Nicolai
    Hähnle)
Signed-off-by: Gert Wollny <gw.fossdev@gmail.com>
Acked-by: Dave Airlie <airlied@redhat.com>