Fix lib/msun/test builds on platforms without 80-bit long doubles

After d3338f3355a612cf385632291f46c5777bba8d18, the lib/msun test case
'hypotl_near_underflow' would fail to compile on platforms where long
doubles weren't 80 bit, like on x86. Disable this particular test on
such platforms for now.

PR: 253313
MFC after: 1 week
X-MFC-With: d3338f3355a612cf385632291f46c5777bba8d18