Add NEON (-mfpu=neon) option to compiler
Fix '-mthumb' linker option

Removing the addition of thumb linker directories since -mthumb is supposed to take care of this (see gcc multilib configuration).
Removing the addition of crtbeginS.o etc. since it's incorrect.
Don't set -nostdlib by default since the NDK doesn'e either
Avoid using ..\.. in linker --sysroot
Add LOCAL_DISABLE_RELRO support
Add ShortWChar compiler property
Add NoExecStack as a general option (affecting both compiler and linker) to mirror LOCAL_DISABLE_NO_EXECUTE.
Remove unused GccToolchainIncludes property.
Initialize GccToolsPath from PrebuiltDir
Major overhaul:
- Unify gcc properties (Gcc.Common.props, gcc.Arm.Common.props).
- Organise Intellisense support by separating into android_gcc_compile.xml (for Intellisense) and android_gcc_compile_tool.xml (for actual compiler).
- Produce preprocessor definition headers for ARM toolchains and features (gcc_features.h) for Intellisense consumption.

Minor changes:
- Remove ANDROID_NDK (since it's not defined by the NDK).
- Add FormatWarnings and FormatSecurityWarnings enumerators.
- Add documentation to some switches.
Case correction
Indentation
Add clang support

- Adding clang 3.4 ARM toolchain
- Fix ToolsetCompilerFlags property
- Unset InlineLimit by default since it has no effect on clang
- Remove superfluous checks for NDK files (and warnings)
Bring back SystemIncludeDirectories and implement switch ordering

Change compiler command line generation to follow the switch order as specified in the XML file. This allows us to place user include directories before the system ones.
Define built-in preprocessor definitions before the user's.
Add linker properties NoWcharSizeWarning and NoEnumSizeWarning
Add forgotten defaults for FormatWarnings and FormatSecurityWarnings
Add libc++ (STL).
Fix include order so that libc++ could override system headers.
Add armv7-a-hard architecture
Reorganizing the properties of include paths for Intellisense and compiler.

From now:
- IntellisenseIncludeDirectories is only for Intellisense
- SystemIncludeDirectories is only for the compiler