[arm] Avoid using negative offsets for 'immediate' addresses when compiling for Thumb2

[arm] Avoid using negative offsets for 'immediate' addresses when compiling for Thumb2

Thumb2 code now uses the Arm implementation of legitimize_address.
That code has a case to handle addresses that are absolute CONST_INT
values, which is a common use case in deeply embedded targets (eg:
void *p = (void*)0x12345678).  Since thumb has very limited negative
offsets from a constant, we want to avoid forming a CSE base that will
then be used with a negative value.

This was reported upstream originally in
https://gcc.gnu.org/ml/gcc-help/2019-10/msg00122.html

For example,

void test1(void) {
  volatile uint32_t * const p = (uint32_t *) 0x43fe1800;

  p[3] = 1;
  p[4] = 2;
  p[1] = 3;
  p[7] = 4;
  p[0] = 6;
}

With the new code, instead of

        ldr     r3, .L2
        subw    r2, r3, #2035
        movs    r1, #1
        str     r1, [r2]
        subw    r2, r3, #2031
        movs    r1, #2
        str     r1, [r2]
        subw    r2, r3, #2043
        movs    r1, #3
        str     r1, [r2]
        subw    r2, r3, #2019
        movs    r1, #4
        subw    r3, r3, #2047
        str     r1, [r2]
        movs    r2, #6
        str     r2, [r3]
        bx      lr

We now get

        ldr     r3, .L2
        movs    r2, #1
        str     r2, [r3, #2060]
        movs    r2, #2
        str     r2, [r3, #2064]
        movs    r2, #3
        str     r2, [r3, #2052]
        movs    r2, #4
        str     r2, [r3, #2076]
        movs    r2, #6
        str     r2, [r3, #2048]
        bx      lr

	* config/arm/arm.c (arm_legitimize_address): Don't form negative
	offsets from a CONST_INT address when TARGET_THUMB2.

From-SVN: r277677