[ARM] Fix several state persistence bugs

[ARM] Fix several state persistence bugs

Summary:
This fixes three bugs, in all of which state is not or incorrecly reset between
objects (i.e. when reusing the same pass manager to create multiple object
files):
1) AttributeSection needs to be reset to nullptr, because otherwise the backend
   will try to emit into the old object file's attribute section causing a
   segmentation fault.
2) MappingSymbolCounter needs to be reset, otherwise the second object file
   will start where the first one left off.
3) The MCStreamer base class resets the Streamer's e_flags settings. Since
   EF_ARM_EABI_VER5 is set on streamer creation, we need to set it again
   after the MCStreamer was rest.

Also rename Reset (uppser case) to EHReset to avoid confusion with
reset (lower case).

Reviewers: rengolin
Differential Revision: http://reviews.llvm.org/D15950

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257473 91177308-0d34-0410-b5e6-96231b3b80d8