Make 'arm' cover both 32 and 64 bit architecutres

... and add aarch32 to specifically refer to the 32-bit ones.

Previously, 'arm' meant only 32-bit architectures and there was no way
for a module to build with both 32 and 64 bit ARM architectures.

Now a module that is intended to work on both architectures can specify
    requires arm
whereas a module only for 32-bit platforms can say
    requires aarch32
and just like before, 64-bit only can say
    requires aarch64

llvm-svn: 244306