[ELF] make default for get{ARM,AArch64}UndefinedRelativeWeakVA unreachable

    
The get{ARM,AArch64}UndefinedRelativeWeakVA() functions should only be
called for PC-relative relocations. Complete the supported pc-relative
relocations in the switch statement and make the default case unreachable.

The R_ARM_TARGET relocation can be evaluated as R_ARM_REL32 but it is only
used in the context of exception tables, and is never output with respect
to a weak reference so it does not appear in the switch statement.
    
Differential Revision: https://reviews.llvm.org/D34138

llvm-svn: 305673