pgsql: For float4/8, remove errno checks for pow() and exp() because

Log Message:
-----------
For float4/8, remove errno checks for pow() and exp() because only some
platforms set errno, and we already have a check macro that detects
under/overflow, so there is no reason for platform-specific code
anymore.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
float.c (r1.135 -> r1.136)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/float.c.diff?r1=1.135&r2=1.136)