[LLD][ELF][ARM] recommit Fix ARM Exidx order for non monotonic section order

Fixed error detected by msan. The size field of the .ARM.exidx synthetic
section needs to be initialized to at least estimation level before
calling assignAddresses as that will use the size field.

This was previously reverted in 1ca16fc4.

Differential Revision: https://reviews.llvm.org/D78422