Allow building plain armeabi binaries for android

Building with "make OS=android APP_ABI=armeabi" will
produce arm binaries that will conform to the armeabi
ABI - not using any features outside of armv5te by
default (but still optionally using the NEON functions at
runtime if detected - even though such devices should rather
use the default armeabi-v7a build).
Use the iMultipleThreadIdc field from SEncParamExt
Use SliceModeEnum as data type for the slice mode fields

This makes the use of the field clearer and safer by allowing
the compiler check that users actually assign proper enum
values.
Support building for any other android architecture

This allows e.g. building for mips, as a plain C++ build with
no assembly.
Explicitly sort the files in mktargets.py

This avoids spurious changes to the targets.mk files when mktargets
is rerun on different platforms and different file systems.

In particular, OS X seems to mostly return files sorted
alphabetically, case insensitively, while they are returned in
a file system specific order on linux.
Merge pull request #455 from mstorsjo/mktargets-sort-files

Explicitly sort the files in mktargets.py
Make the processing lib use mutexes from WelsThreadLib from the common library

This requires always building the WelsMutex* functions,
even if MT_ENABLED isn't set.
Add NO_DYNAMIC_VP when building the encoder lib for the iOS simulator

This was previously only added to builds for actual devices.
Enable threading when building for iOS using the project files

This was already enabled when building the library for iOS using
the makefiles.
Merge pull request #457 from mstorsjo/ios-enc-defines

Add missing defines when building the encoder for iOS using the xcode project files
Don't try to set thread scope and scheduling policy on android

These APIs aren't implemented on android.
Set the arch to armv7 by default when building for iOS

Otherwise it would build for the simulator by default, which
in most cases probably isn't what was intended.
Mark pointers as const where possible in vaacalc
Remove unnecessary stray __cdecl annotations in function signature comments in x86 assembly
Remove a stray inline keyword in a function signature comment in x86 assembly

Assembly functions written in external assembly files is obviously
not inlined.
Fix a typo, "heigth" -> "height"
Fix a typo in arm assembly, LORD -> LOAD
fix WELS_READ_VERIFY
Merge pull request #464 from licaiguo/fix-type-wels-read-verify

fix WELS_READ_VERIFY
Merge pull request #461 from mstorsjo/fix-asm-comments

Remove unnecessary/stray keywords in function signature comments in x86 assembly