Fill ROM with jump instructions
Jump instructions are PC-relative on Cortex M and would need 32 bits unless your flash is really tiny. 

A better idea would be the "permanently UNDEFINED" instruction code 0xDE00 or 0xDEDE - all 16-bit 0xDEXX instructions would trigger the UsageFault handler function. See Output Section Fill in the LD manual.

Personally I would not bother:


Without the Thumb-Bit all even addresses automatically trigger a fault while being loaded into the PC
A large part of the 32 bit address space is not covered by the flash memory
The PC should not be "randomly" modified unless we're talking about SEU in a highly radioactive environment