[MC][test] Test Mach-O __TEXT,__gcc_except_tab

This test is simplified from
lld/test/MachO/compact-unwind-lsda-folding.s, which tests .uleb128 A-B
where A and B are in different fragments (not tested in llvm/).

`.uleb128 Lfunc_end0-Ltmp1` requires evaluateKnownAbsolute in
MCAssembler::relaxLEB to be foldable.