Indent nested ifdefs in the android makefile

This makes the makefile slightly more readable.
Use an intermediate platform header for including architecture specific rules

This avoids coupling the architecture directly to the platform,
allowing building on e.g. linux on architectures other than x86,
without having to explicitly specify USE_ASM=No while building
on those.

Later this will also facilitate enabling assembly optimizations
where applicable, e.g. arm assembly on linux.
Allow using the USE_ASM makefile variable for architectures other than x86

Add an ASM_ARCH variable which specifies which kind of assembly
is supposed to be built.
Don't explicitly set/clear USE_ASM in the android makefile

The USE_ASM variable is architecture independent now and only
controls whether assembly optimizations should be enabled in
the general sense, regardless of the target architecture.
Don't add x86 assembly specific options on darwin if building for other archs

This avoids warnings about using the -read_only_relocs suppress
if building for other architectures than x86.
Add a platform file for building for iOS using the makefile build system

This doesn't build the actual demo apps that you can run on the
phone (not yet at least), but is useful for easily testing compilation
in different setups, and for building a precompiled static library for
use in external projects.
Move the ASM variable declaration into the x86-common file
Merge pull request #364 from mstorsjo/build-ios-make

Decouple architecture (x86) from platforms in the make build system