Introduce runtime optimizations for statistical computations per PR10234.

Introduce runtime optimizations for statistical computations per PR10234.

This update improves the performance of systemtap runtime statistical
computations by optimizing out unneeded parts of the __stp_stat_add()
function.  It is based on parametrizing and inlining it.  The stap
translator now generates _stp_stat_add(), or _stp_pmap_add_*() calls
that have additional "optimization" parameters respective to stats
in use for given global.  GCC uses this for optimizing the inlined
__stp_stat_add() calls.

The optimization effect significantly depends on compiler version,
platform architecture, and the stat operators being used for given
global.  At the moment, the available stat operators are @count,
@sum, @min, @max, @avg, and @variance. The most computionally
expensive is @variance.  The effect of optimizing @variance is
significant.  Other stat operators are computionally chap and so
the effect of their optimizations is relatively low.

Using gcc-6.2.1-1.fc26.x86_64, the @count, @sum, @min, and @max
optimizations brings approximately 8% run time shrinkage. The
@variance optimization shrinkage is up to 70% using this compiler.
For other architectures, namely for power, the optimization is less
effective.

runtime/map-gen.c: Pass the additional optimization parameters through
	the map API generator macros.
runtime/map.c: Modify _new_map_set_stat() to accept optimization
        additional parameters.
runtime/map.h: Ditto.
runtime/pmap-gen.c: Pass the additional optimization parameters to
	__stp_map_set*() and to _stp_pmap_add*().
runtime/stat-common.c: Add optimization params to __stp_stat_add().
runtime/stat.c: Add optimization params to _stp_stat_add().
tapsets.cxx: Generate parametrized calls to the runtime.
translate.cxx: Ditto.
testsuite/systemtap.base/optim_stats*: New testcase.