testsuite: Only run recomp015 on ELF-based platforms

Authored by bgamari.

Description

testsuite: Only run recomp015 on ELF-based platforms

It fails on OS X with hundreds of messages of the form,

ManySections.s:196576:10: error:
     error: mach-o section specifier uses an unknown section type
.section s65525,"",@progbits
         ^

ManySections.s:196579:10: error:
     error: mach-o section specifier uses an unknown section type
.section s65526,"",@progbits

It fails on Windows with messages of the form,

ManySections.s:196579:10: error:
     Error: junk at the end of line, first unrecognized character is ','

Test Plan: Validate

Reviewers: hsyl20, thomie, austin

Reviewed By: thomie, austin

Differential Revision: https://phabricator.haskell.org/D1601

GHC Trac Issues: Trac #11022