Enable long double tests on RISC-V

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
    long double LD;
    ASSERT(sizeof(LD) > sizeof(uint64_t));
#endif

RISC-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.

Reviewed by: ngie
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25419