The fix is suggested by https://hackerone.com/lucnguyen

The issue is reported by https://hackerone.com/aerodudrizzt

Internal method_missing works without problems.

GitLab.com tests for multiple configurations

Build with byacc

Additional options for Android build script.

Android toolchain separated target architecture compile flags (ctarget) from shared compile flags (cflags). Added support for custom mfpu and float-abi switches for the armeabi-v7a target.

Build fix for 32-bit clang 3.8/3.9 with MRB_INT64

Gitlab testing is introduced, using GitLab CI, to test many different configurations:
 - 32/64 bit architecture
 - float/double
 - 16/32/64 bit int size
 - none/NaN/word boxing
 - ASCII/UTF8

using various compilers:
 - gcc-4.7
 - gcc-4.8
 - gcc-4.9
 - gcc-5
 - gcc-6
 - clang-3.5
 - clang-3.6
 - clang-3.7
 - clang-3.8
 - clang-3.9

CRuby defines #instance_exec in BasicObject, but we don't.
It's a small incompatibility that isn't worth accomplish at
the price of implementation complexity.

More compatibility to CRuby.
Updated tests that assume old mruby behavior.

`mrb_funcall()` recursion can cause stack overflow easily,
so recursion depth is now limited to MRB_FUNCALL_DEPTH_MAX,
which default value is 512.

This change reduce flexibility but makes mruby simpler and faster.

Update android build script.