Fix :hexadecimal_floating_point on i386

Don't exclude i386 from LDBL_MANT_DIG == 64; it works properly in
that case.

While here, replace strcmp + atf_tc_fail with ATF_CHECK_MSG for 2
reasons:

• Gather as many results as possible instead of failing early and not testing the rest of the cases.
• Simplify logic when checking test inputs vs outputs and printing test result.

Tested on: amd64, i386
MFC after: 1 week
Sponsored by: Dell EMC Isilon