tests/lib/libc/gen/t_isnan: Fix architecture conditionals.

1. Skip isinf_basic on VAX. We have no other way to query `does this
architecture support infinities?' and other tests already use
isinf(HUGE_VAL) to determine that, so let's just make sure it
works correctly on VAX.

2. Don't skip these tests on m68k. This was originally conditional
on m68000 (which is to say, NetBSD's MACHINE_ARCH for Motorola
68010 CPUs, i.e., the sun2 port), and was later mistakenly changed
to m68k. But there's no reason any m68k, 68010 or later, should
skip this -- the floating-point format of the ABI does have NaN
and infinity, whether it's a softfloat ABI or hardfloat ABI.

PR misc/59266: t_isnan:isinf_basic test is bogus on some ports

branches: 1.5.28;
PR misc/49356 remove unnecessary references to atf-c/config.h

The function included via this header is not used and is removed in
later versions of atf, so let us avoid it.

Use compiler builtins instead of atf_arch and atf_machine.

The atf_arch and atf_machine configuration variables were removed from
atf-0.19 without me realizing that some tests were querying them directly.

Instead of reintroducing those variables, just rely on compiler builtins
as many other tests already do.

Should fix PR bin/48582.

Retry previous

Make it compile on archs where NAN is not defined - previously it only
compiled by chance (and details of the __isnan macro) on vax.

branches: 1.1.2; 1.1.8;
Move the tests/libc/ieeefp to tests/libc/gen to match the structure of libc.
Also rename the test files to gain functional scope.