[counters] Add system counters

After discussion with Kumpera, it was decided to load them lazily when calling mono_counters_foreach for the first time. It has to be inside the mono runtime, and not in a profiler because it accesses proclib API which is all MONO_INTERNAL functions.
[counters] Edit get process pid and /proc/loadavg according to Paolo review
[sdb] Fix the serialization of variables with address mode VTADDR.
[jit] Add a test for lrem_un.
[sdb] Avoid placing implicit sequence points at offsets where the IL stack is empty when using symbol file based sequence points. Fixes #13640.
[sdb] Fix a deadlock in the assembly cache invalidation code.
[system.net.http] Fix encoding of Content-MD5. Based on patch by nberardi. Fixes #19644
[http] Convert string output data using BOM header when available. Fixes #19667
[decode] Add check for computer endianness on startup
[decode] Add Counter, CounterValue, CounterList, CounterSection and CounterTimestamp structs

Counter : contains metadata concerning a counter. There is one in memory per counter index (right now ~100).
CounterValue : represent a sample value of a counter.
CounterList : it is a utility struct that contains a list of counters.
CounterSection : represent all the counters that are in this sections. Used only when we sort counters per category.
CounterTimestamp : represent all the counters that have a sample value at this timestamp. Used only when we sort counters per time.
[decode] Add functions to manipulate counters, counters_sections and counters_timestamps

This variables are respectively :
 - the list of all counters
 - the list of all counters grouped by sections
 - the list of all counters groupes by sample value timestamps and sections
[decode] Add functions to transform mono-counters enum to strings
[decode] Add parsing for TYPE_SAMPLE_COUNTERS_DESC
[decode] Add parsing for TYPE_SAMPLE_COUNTERS
[decode] Add dump_counters

Two main way to dump :
 - per Time : equivalent to a --stats but every seconds, useful to see the overall evolution of all counters
 - per Category : print sample values grouped per category and counters, useful to see the evolution of a specific counter over time
[decode] Add command line counters options parsing
[decode] Fix sample difference parsing
[decode] Edit according to Paolo review
[arm] Add support for the new compare instructions used by native types.
[system.web] remove some more remnants of Mono.Web.dll to fix Xamarin Studio activation

The section mapping system wasn't being initialized and used properly since some ifdefs had
been left in the source code. This caused a null reference exception while trying to perform
some HTTP requests and, as the result, broke Xamarin Studio activation.