Remove a dubious/unnecessary workaround for an issue in a nonstandard toolchain

There's no requirement to work with the Intel NDK. The NDK build
files don't even need to create any cpufeatures subdirectory since
all of the cpufeatures library is handled within the normal build
system. Finally, the encoder makefile tried to create a directory
for "welsdecdemo", not anything for "welsencdemo".

Thus, if this really was necessary, it would already have been noticed
that it missed an entry for "welsencdemo". Therefore, remove this
hack/workaround.
Merge pull request #471 from ruil2/encoder_slice

fix SM_FIXEDSLCNUM_SLICE bug, add SM_AUTO_SLICE mode
Merge pull request #472 from mstorsjo/android-remove-mkdir-workaround

Remove a dubious/unnecessary workaround for an issue in a nonstandard toolchain
Mark the stack as non-executable in the arm assembly

Otherwise the linker is forced to enable an executable stack for
executables that the code is linked into.
Remove something that looks like a personal todo note from the Android makefiles
Merge pull request #474 from mstorsjo/remove-todo-comment

Remove something that looks like a personal todo note from the Android makefiles
Merge pull request #473 from mstorsjo/arm-non-executable-stack

Mark the stack as non-executable in the arm assembly
modify the parameter verification for SM_AUTO_SLICE mode -- uiSliceNum
is ignored