Reverting gcc-4.3 doesn't help matters.
Now this DF bit flaw is known, then anyone can patch a compiler or use
assembler to attempt to craft an exploit in application code. The kernel
is broken, it should not rely on called code to clear the flag, but
actually ensure the registers are saved, set & restored according to ABI.
Perhaps kernels compiled with gcc < 4.3, can rely on gcc clearing the flag
when the signal handler is called in a subroutine, in which case it is
reasonable to argue that back-porting a fix to support gcc-4.3 may not be
absolutely necessary. But it's probably as simple to patch stable kernel
updates with the fix, as it is to detect and warn about a build using 4.3.
It's not admin-friendly to rely on older kernel source not being built
with the latest gcc.
Past experience with "apparently unexploitable" flaws, tends to suggest
that correcting the code is the only safe option.