ldbl-128ibm-compat: Add tests for IBM long double functions

This patch creates test-ibm128* tests from the long double function tests.
In order to explicitly test IBM long double functions -mabi=ibmlongdouble is
added to CFLAGS.

Likewise, update the test headers to correct choose ULPs when redirects
are enabled.

Co-authored-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>
Co-authored-by: Paul E. Murphy <murphyp@linux.vnet.ibm.com>