First round of trying to improve the performance of attr_reader and attr_writer. From ~24sec to ~15sec
rename advance_phase to incremental_gc_until

in fact advance_phace() is not really a "phase", but can be used
as an utility function.
cleanup redundant codes with incremental_gc_until()
Wrote tests around attr, attr_reader, attr_writer, attr_accessor
Merge branch 'master' into attr-perf-fix

Conflicts:
	mrblib/class.rb
Improve Android NDK toolchain support.

Linux-x86_64 has not been supported now.
This commit add Linux-x86_64 platform support.
Fail if dependency isn't found
Slight cleanup of code
Changed the attr methods in mrblib, so that they take advantage of validating the name against a string parameter if need be
Merge branch 'master' into attr-perf-fix
Reset the so-render benchmark so that it is what it was before my attr performance testing.
Cleaned up some of the Module#attr tests
Merge pull request #1393 from crimsonwoods/add-android-ndk-linux-x86_64-support

Improve Android NDK toolchain support.
Merge pull request #1395 from carsonmcdonald/fixdepcheck

Fail if dependency isn't found
Merge pull request #1396 from Archytaus/attr-perf-fix

Performance improvement of methods generated by Module#attr
change else formatting
Merge pull request #1392 from Fleurer/gc-rename-advance-phase

rename advance_phace() to incremental_gc_until(), and cleanup some redundant codes with it.
revise the Two White Parts comments in gc.c
Merge pull request #1397 from Fleurer/revise-comments-on-white-colors

revise the Two White Parts comments in gc.c