[mach-o] for 32-bit arm, in -r mode support B (not BL) to undefined symbol

When fixing up BL instructions, the linker has to compare the thumbness of the
target to decide if the instruction needs to be converted to BLX.  But with B
instruction there is no BX, so the linker asserts if the target is not the
same thumbness. This assert was firing in -r mode when the target was undefined
which it interpreted as being non-thumb.

Test case change is to add a B (in both thumb and arm code) to an undefined
symbol and round trip through -r mode.