[libc++abi] Apply simplify scan_eh_tab to SjLj

Previous "simplify scan_eh_tab" patch, https://reviews.llvm.org/D93190,
saves landingpad if and only if the target is not using SjLj exceptions.
However, the landingpad is used by SjLj exception handler also.  This
patch changes to set landingpad for both exception handlers.

Differential Revision: https://reviews.llvm.org/D108082

(cherry picked from commit 9d2740f3)