Disable test-ctrig.t testcases which fail assertions on i386 [*]

Also, don't compile the exp2l(3) testcases on platforms that don't support the
libcall (technically only x86 right now). This makes this test buildable on
arm*/mips*/powerpc*

Tested on: stable/10 (amd64/i386), head (amd64/i386)
PR: 205446 [*]
MFC after: 1 week
X-MFC to: stable/10
Sponsored by: EMC / Isilon Storage Division