Small problem compiling Loom with GCC 10.1.0

[Mark Raynsford]

Hello!

I see the following with the latest fibers branch:

* For target support_native_java.base_libfdlibm_k_standard.o:
/home/rm/git/com.github/openjdk/loom/src/java.base/share/native/libfdlibm/k_standard.c: In function '__j__kernel_standard':
/home/rm/git/com.github/openjdk/loom/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;
      |                ~~~^~~~~~~
cc1: all warnings being treated as errors

It's not wrong. If the function is passed some "type" value outside
of the range [1,42], the function will return whatever happened to be
on the stack in exc.retval. Perhaps adding an assertion would make
sense here? 

-- 
Mark Raynsford | https://www.io7m.com