[LLDB] Fix PDB/pointers.test for 32bit Arm/Windows

PDB/pointers.test was orignally written for 32bit x86 keeping in mind
__cdecl and __stdcall calling conventions which does name mangling for
example like adding "_" underscore before function name.
This is only x86 specific but purpose of pointers.test is NOT to test
calling convention.
I am have made a few minor changes to this test which will make it pass
when run on Windows/Arm platform.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D128668