Folding a constant into the instruction
Adding a small constant costs no extra load. agbcc folds the number straight into the instruction with the immediate form add Rd, Rd, #imm — #imm being the literal value carried inside the opcode. A function that adds twelve to its argument compiles to this:
add r0, r0, #0xc
bx lr
Notice the constant is written in hexadecimal: #0xc is decimal 12 — agbcc's -fhex-asm habit, and every immediate you meet here will be in hex, so get comfortable converting. The Thumb immediate field is eight bits, reaching 0 to 255; ask for more and the compiler has to build the value some other way — but that won't happen in this exercise.
Read the constant the target add carries — in hex — and that is the number you are after.
Your task
Write bias, taking an int x, so it compiles to the target add instruction.