For the ARM, ld will generate code stubs to allow functions calls betweem ARM and Thumb code. These stubs only work with code that has been compiled and assembled with the -mthumb-interwork command line option. If it is necessary to link with old ARM object files or libraries, which have not been compiled with the -mthumb-interwork option then the -support-old-code command line switch should be given to the linker. This will make it generate larger stub functions which will work with non-interworking aware ARM code. Note, however, the linker does not support generating stubs for function calls to non-interworking aware Thumb code.
The -thumb-entry switch is a duplicate of the generic -entry switch, in that it sets the program's starting address. But it also sets the bottom bit of the address, so that it can be branched to using a BX instruction, and the program will start executing in Thumb mode straight away.