[ELF][ARM] Refine check for when undefined weak needs a Thunk

When an undefined weak reference has a PLT entry we must generate a range
extension thunk for any B or BL that can't reach the PLT entry.

This change explicitly looks for whether a PLT entry exists rather than
assuming that weak references never need PLT entries unless Config->Shared
is in operation. This covers the case where we are linking an executable
with dynamic linking, hence a PLT entry will be needed for undefined weak
references. This case comes up in real programs over 32 Mb in size as there
is a B to a weak reference __gmon__start__ in the Arm crti.o for glibc.

Differential Revision: https://reviews.llvm.org/D40248

llvm-svn: 319020