Update predefined versions

There wasn't much feedback on the newsgroup so I've implemented the suggested changes. Please give some feedback if those changes are OK.

* Consistent naming: SoftFP --> SoftFloat (on ARM SoftFP != SoftFloat)
                     HardFP --> HardFloat
* DOC: ARM_Thumb means any Thumb version
* ARM/ARM64 fix reference regarding AARCH (AARCH is an arm8 term
  but ARM is also defined for ARM7,6,5...)
* Remove MIPS: Use MIPS32 instead
* Remove MIPS_NoFloat: This should be a generic version D_NoFloat
  or it shouldn't be defined at all.
* D_X32: Add a note that defining both D_X32 and X86_64 is
valid/expected
* D_LP64: We only talk about pointers, this is not the C LP64 data
model!