[ARM] Improve comment on the 'J' inline asm modifier. (#160712)

An inline asm constraint "Jr", in AArch32, means that if the input value
is a compile-time constant in the range -4095 to +4095, then it can be
inserted into the assembly language as an immediate operand, and
otherwise it will be placed in a register.

The comment in the Arm backend said "It is not clear what this
constraint is intended for". I believe the answer is that that range of
immediate values are the ones you can use in a LDR or STR instruction.
So it's suitable for cases like this:

asm("str %0,[%1,%2]" : : "r"(data), "r"(base), "Jr"(offset) : "memory");

in the same way that the "Ir" constraint is suitable for the immediate
in a data-processing instruction such as ADD or EOR.

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