Recognize -1 as a tombstone value in .debug_line

LLD from 11 onwards (https://reviews.llvm.org/D81784) uses -1 to
represent a relocation in .debug_line referencing a discarded symbol.
Recognize -1 to fix gdb.base/break-on-linker-gcd-function.exp when the
linker is a newer LLD.

gdb/ChangeLog:

	* dwarf2/read.c (lnp_state_machine::check_line_address): Test -1.