Some of the NetBSD contributed tests are gated behind the
__HAVE_LONG_DOUBLE flag. This flag seems to be defined only for
platforms whose long double is larger than their double. I could not
find this explicitly documented anywhere, but it is implied by the
definitions in NetBSD's sys/arch/${arch}/include/math.h headers, and the
following assertion from the ubsan code:
#ifdef __HAVE_LONG_DOUBLE
ASSERT(sizeof(LD) > sizeof(uint64_t));
#endifRISC-V has 128-bit long doubles, so enable the tests on this platform,
and update the comments to better explain the purpose of this flag.