Avoid calls to realloc for nvptx

[Bernd Schmidt]

This is a followup patch for the nvptx port. Since malloc and free are 
magically provided by the ptx environment, but realloc is missing, it's 
nontrivial to provide an implementation for it. The Fortran frontend 
likes to generate calls to realloc, but in one case it seems like we can 
compute the old size, and call a function that does malloc/memcpy/free 
instead.

It seems desirable to continue calling plain realloc on most targets, 
but the decision which function to use must happen after the LTO stage 
for it to work when ptx is used as an offload target. The following 
patch provides a new builtin function which gets expanded to one of the 
two alternatives based on a target macro, and modifies the Fortran 
frontend to call the new builtin instead of using realloc directly when 
the old size is known.

Bootstrapped and tested on x86_64-linux. Two oddities: the last test run 
had some plugin tests failing due to a version check; this went away 
when I recompiled gcc/plugin.c - it doesn't seem like the sort of thing 
that would be caused by this patch and I didn't see it in any of the 
earlier test runs.

The other oddity is that the order of args when realloc is defined by 
f95-lang seems reversed. However, fixing this causes infinite loops in 
the fre1 pass for some Fortran tests so I've left it as-is for now.

Ok once the nvptx port is in?


Bernd
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