xfail gdb.base/break-on-linker-gcd-function.exp with Solaris ld [PR34534]

As detailed in PR testsuite/34534, a test FAILs on Solaris when using
the native ld:

FAIL: gdb.base/break-on-linker-gcd-function.exp: break on line in garbage collected function
FAIL: gdb.base/break-on-linker-gcd-function.exp: readnow: break on line in garbage collected function

Even though ld supports --gc-sections as an alias for the native -z
discard-unused=sections, this doesn't work in this particular case.
While the issue is under investigation, this patch xfail's the test when
Solaris ld is in use.

Approved-By: Tom Tromey <tom@tromey.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34534