[Bug c/94200] New: -mabi=ibmlongdouble and -mlong-double-128 produces error

[murphyp at linux dot vnet.ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94200

            Bug ID: 94200
           Summary: -mabi=ibmlongdouble and -mlong-double-128 produces
                    error
           Product: gcc
           Version: 7.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: murphyp at linux dot vnet.ibm.com
  Target Milestone: ---
            Target: powerpc64le

When compiling an empty file with:

`touch foo.c; gcc -mabi=ibmlongdouble -mlong-double-128 foo.c`

produces the following:

gcc: warning: using IBM extended precision long double
cc1: warning: using IBM extended precision long double
cc1: error: ‘-mabi=ibmlongdouble’ requires ‘-mlong-double-128’

Notably, removing the option -mlong-double-128 does not trigger the above.

This was fixed in GCC 8, and does not happen with GCC 7.4.0