[llvm] [JITLink][AArch64] Implement R_AARCH64_LDR_PREL_LO19 (PR #82172)

[Eymen Ünay via llvm-commits]

================
@@ -51,6 +51,21 @@ test_adr_prel_lo21:
 ## to test this bit better
 adr_data = test_adr_prel_lo21 + 0xe46f2
 
+# Check R_AARCH64_LD_PREL_LO19 relocation of a local symbol
----------------
eymay wrote:

I think we can have a more simple test case. I have sketched this which is passing the tests:
```
-# jitlink-check: decode_operand(test_ldr_prel_lo19 + 12, 1)[19:0] = \
-# jitlink-check:     (ldr_data - test_ldr_prel_lo19_inst + 0x4)[21:2]
-        .globl  test_ldr_prel_lo19, test_ldr_prel_lo19_inst, ldr_data
+
+# jitlink-check: decode_operand(test_ldr_prel_lo19 + 0, 1)[19:0] = \
+# jitlink-check:  (ldr_data - test_ldr_prel_lo19 + 0x4)[21:2]
+        .globl  test_ldr_prel_lo19, ldr_data
         .p2align  2
 test_ldr_prel_lo19:
-        nop
-        nop
-        nop
-test_ldr_prel_lo19_inst:
         ldr    x0, ldr_data + 0x4
         .size test_ldr_prel_lo19, .-test_ldr_prel_lo19
 ldr_data = test_ldr_prel_lo19 + 4
```

https://github.com/llvm/llvm-project/pull/82172