Skip the testcases on i386 (all the assertions fail) [*]

Also, don't compile the ldexpl(3) testcases on platforms that don't support
the libcall (technically only x86 right now). This makes this test buildable on
arm*/mips*/powerpc*

PR: 205449 [*]
MFC after: 1 week
Tested on: stable/10 (amd64/i386), head (amd64/i386)
Sponsored by: EMC / Isilon Storage Division