This is a follow-up to commit fef67987cf502fe322e92ddce22eea7ac46b4d75:

https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=fef67987cf502fe322e92ddce22eea7ac46b4d75

I realized last week that having multilib-specific versions of 
ISO_Fortran_binding.h (generated by running the compiler to ask what 
kinds it supports) was still broken outside of the test support; the 
directory where it's being installed isn't on GCC's normal search path. 
It seemed to me that it was better to try to find some other solution 
for this problem than to venture down what appears to be a rat hole.

I've come up with this patch to return to a single ISO_Fortran_binding.h 
file that uses preprocessor magic to identify the Fortran kind 
corresponding to the standard C long double type and the GCC extension 
types __float128 and int128_t.  I haven't attempted to undo the 
follow-up patches that fixed in-tree testing; the static .h file is 
still copied to the build directory, and it can still be referenced with 
<> syntax during testing.

Any complaints about either the overall strategy here, or the logic to 
infer the C type -> kind mapping?  Or OK to commit?

-Sandra