Instead of shifting mruby VM stack, we always use CALL_MAXARGS.

There are 3 levels of C++ exception handling:
* default - no C++ exception (use setjmp/longjmp)
* enable_cxx_exception (use C++ exceptions with C ABI)
* enable_cxx_abi (use C++ ABI including exceptions)

ref #3267 #3470

By this commit, mruby do not use C++ ABI mode unless
you specify explicitly. It compiles C files by C
compilers, with C++ exception enabled when it sees
C++ files in your configured mrbgems.

I haven't tried visualcpp, so please submit an issue
if you see any problem with C++ gems on Windows.

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