ARM: gmp has no support for thumb interworking

[Richard Earnshaw]

The hand-coded assembler functions in GMP have no support for being built 
with interworking as required by the ARM EABI.  This makes it impossible 
to link libgmp with any code compiled in Thumb state, and thus now 
prevents me from testing GCC development builds by bootstrapping in Thumb 
code.

The functions also unconditionally assume the presence of ARM architecture 
4.  Again this means that it is no longer possible to build a native 
version of GCC that will run on architecture 3 or earlier parts.

R.