[MC][ARM] Don't create multiple .ARM.exidx associated to one .text

Fixed an issue exposed by D74006.

In clang cc1as, MCContext::UseNamesOnTempLabels is true.
When parsing a .fnstart directive, FnStart gets redefined to a temporary symbol of a different name (.Ltmp0, .Ltmp1, ...).
MCContext::getELFSection() called by SwitchToEHSection() will create a different .ARM.exidx each time.

llvm-mc uses `Ctx.setUseNamesOnTempLabels(false);` and FnStart is unnamed.
MCContext::getELFSection() called by SwitchToEHSection() will reuse the same .ARM.exidx .

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D75095