lib/msun: avoid referring to broken LDBL_MAX

LDBL_MAX is broken on i386:
https://lists.freebsd.org/pipermail/freebsd-numerics/2012-September/000288.html

Gcc has produced +Infinity for LDBL_MAX on i386 and amd64 with -m32
for some time, and newer versions of gcc are now warning that the
"floating constant exceeds range of 'long double'". Avoid this by
referring to half the value of LDBL_MAX instead.

Reviewed by: bde
Approved by: markj (mentor)
Sponsored by: Dell EMC Isilon