arm: Handle undefweak with ST_BRANCH_UNKNOWN

A previous patch made ld fail early on Thumb-only where branch_type is
ST_BRANCH_UNKNOWN.

However, this fails erroneously when the target is undefweak: in that
case the branch should be replaced by a branch to the next instruction
(or nop.w on thumb2).  This patch accepts this case and restores the
previous behaviour in such cases.

This was reported by failures in the GCC testsuite, where we fail to
link executables because __deregister_frame_info is undefweak:

(__deregister_frame_info): Unknown destination type (ARM/Thumb) in ...crtbegin.o
crtbegin.o: in function `__do_global_dtors_aux':
crtstuff.c:(.text+0x52): dangerous relocation: unsupported relocation