Ignore linker script loading on OpenBSD

Starting with e020e387 which addressed #2615 (closed) ghci had support for loading linker scripts as .so. This capability is verified with T2615.hs test. The test fails on OpenBSD because the regexp looking for this condition is not sufficient to match the error message returned by OpenBSD ld.so. The small attached program (extracted out of Linker.c)l.c demonstrates the condition and its output is:

errmsg = 'File not an ELF object'
result = 1
result File not an ELF object

The re_invalid could conceivably be expanded to match this error as well, but the underlying assumption that the file name is included into dlerror result does NOT hold on OpenBSD.

Luckily, OpenBSD doesn't ship any system libraries fronted by linker scripts. Hence the only impact at the moment is the broken test which I'll mark to be ignored.