[PPC] Fix PR24686 by failing assembly for an invalid relocation

PR24686 identifies a problem where a relocation expression is invalid
when not all of the symbols in the expression can be locally
resolved.  This causes the compiler to request a PC-relative half16ds
relocation, which is nonsensical for PowerPC.  This patch recognizes
this situation and ensures we fail the assembly cleanly.

Test case provided by Anton Blanchard.

llvm-svn: 251027