[ARM] Fix handling of mapping symbols when changing sections

ChangeSection incorrectly registers LastEMSInfo as belonging to the previous
section, not the current section. This happens to work when changing sections
using .section, as the previous section is set to the current section before
the call to ChangeSection, but not when using .popsection.

Differential Revision: https://reviews.llvm.org/D32225

llvm-svn: 300831