_IEEE_LIBM targets and __kernel_standard

[Florian Weimer]

__kernel_standard in src/java.base/share/native/libfdlibm/k_standard.c
is built for _IEEE_LIBM targets as well, but it appears superfluous
there.

In noticed this because GCC 10 flags an uninitialized variable in this
code:

…/src/java.base/share/native/libfdlibm/k_standard.c: In function 
'__j__kernel_standard':
…/src/java.base/share/native/libfdlibm/k_standard.c:743:19: error: 'exc.retval' 
may be used uninitialized in this function [-Werror=maybe-uninitialized]
  743 |         return exc.retval;
      |                ~~~^~~~~~~

Rather than debating whether this warning is reasonableor not, I would
suggest to #ifdef out this code for _IEEE_LIBM targets (are there any
else left?).

Thanks,
Florian