Update libm tests from NetBSD

I did this without a full vendor update since that would cause too many
conflicts. Since these files now almost match the NetBSD sources the
next git subtree merge should work just fine.

Reviewed By: lwhsu
Differential Revision: https://reviews.freebsd.org/D28797

Un-XFAIL two tests with Clang > 10

SVN r343917 fixed this for in-tree clang, but when building with a newer
out-of-tree clang the test was still marked as XFAIL.

Reviewed By: dim
Differential Revision: https://reviews.freebsd.org/D28390

Amend r343442, by only expecting the lib.msun.cbrt_test.cbrtl_powl and
trig_test.reduction test cases to fail, if the fixes from r343916 have
not yet been applied to the base compiler.

Reported by: lwhsu
PR: 234040
Upstream PR: https://bugs.llvm.org/show_bug.cgi?id=40206
MFC after: 1 week

Temporarily mark lib.msun.{cbrt_test.cbrtl_powl,trig_test.reduction}
expected failure after clang700-import merge

PR: 234040
Reviewed by: ngie, markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18938

- Skip over the testcases that call cbrtl on platforms where LDBL_PREC == 53
(arm, mips, powerpc). This fixes the build on these platforms, based on some
ad hoc tinderbox runs I did a while ago
- Skip cast the arguments to powl as long double so powl properly interprets
those arugments at compile-time when picking the type

Sponsored by: EMC / Isilon Storage Division