PR27228, .reloc wrong symbol emitted for undefined local symbol

Local symbols are of course supposed to be defined by their object
file, but in other cases a local symbol is promoted to global by gas
if undefined and referenced.  This patch stops gas wrongly replacing a
local undefined symbol with the undefined section symbol, resulting in
a .reloc undefined local symbol being emitted as global.

	PR 27228
	* write.c (resolve_reloc_expr_symbols): Don't assume local symbol
	is defined.

(cherry picked from commit 68fcee4f)