Add _GLOBAL_OFFSET_TABLE_ support

This adds lookup logic for _GLOBAL_OFFSET_TABLE_ as well as
relocation logic for R_ARM_BASE_PREL and R_ARM_GOT_BREL which
the gnu toolchain (gas, gcc, ...) prefers to produce.  Apparently
recent llvm toolchains will produce those as well.

rts/linker/Elf.c

@@ -1019,6 +1019,19 @@ do_Elf_Rel_relocations ( ObjectCode* oc, char* ehdrC,
        return 1;
    }
 
+   /* The following nomenclature is used for the operation:
+    * - S -- (when used on its own) is the address of the symbol.
+    * - A -- is the addend for the relocation.
+    * - P -- is the address of the place being relocated (derived from r_offset).
+    * - Pa - is the adjusted address of the place being relocated, defined as (P & 0xFFFFFFFC).
+    * - T -- is 1 if the target symbol S has type STT_FUNC and the symbol addresses a Thumb instruction; it is 0 otherwise.
+    * - B(S) is the addressing origin of the output segment defining the symbol S. The origin is not required to be the
+    *        base address of the segment. This value must always be word-aligned.
+    * - GOT_ORG is the addressing origin of the Global Offset Table (the indirection table for imported data addresses).
+    *        This value must always be word-aligned.  See §4.6.1.8, Proxy generating relocations.
+    * - GOT(S) is the address of the GOT entry for the symbol S.
+    */
+
    for (j = 0; j < nent; j++) {
        Elf_Addr offset = rtab[j].r_offset;
        Elf_Addr info   = rtab[j].r_info;
@@ -1113,19 +1126,35 @@ do_Elf_Rel_relocations ( ObjectCode* oc, char* ehdrC,
 #        endif
 
 #        ifdef arm_HOST_ARCH
-       case COMPAT_R_ARM_ABS32:
+       case COMPAT_R_ARM_ABS32:     /* (S + A) | T */
            // Specified by Linux ARM ABI to be equivalent to ABS32
        case COMPAT_R_ARM_TARGET1:
            *(Elf32_Word *)P += S;
            *(Elf32_Word *)P |= T;
            break;
 
-       case COMPAT_R_ARM_REL32:
+       case COMPAT_R_ARM_REL32:     /* ((S + A) | T) – P */
            *(Elf32_Word *)P += S;
            *(Elf32_Word *)P |= T;
            *(Elf32_Word *)P -= P;
            break;
 
+       case COMPAT_R_ARM_BASE_PREL: /* B(S) + A – P */
+       {
+           int32_t A = *pP;
+           // bfd used to encode sb (B(S)) as 0.
+           *(uint32_t *)P += 0 + A - P;
+           break;
+       }
+
+       case COMPAT_R_ARM_GOT_BREL: /* GOT(S) + A – GOT_ORG */
+       {
+           int32_t A = *pP;
+           void* GOT_S = symbol->got_addr;
+           *(uint32_t *)P = (uint32_t) GOT_S + A - (uint32_t) oc->info->got_start;
+           break;
+       }
+
        case COMPAT_R_ARM_CALL:
        case COMPAT_R_ARM_JUMP24:
        {

rts/linker/elf_got.c

@@ -86,9 +86,13 @@ fillGot(ObjectCode * oc) {
                     if(0x0 == symbol->addr) {
                         symbol->addr = lookupSymbol_(symbol->name);
                         if(0x0 == symbol->addr) {
-                            errorBelch("Failed to lookup symbol: %s\n",
-                                       symbol->name);
-                            return EXIT_FAILURE;
+                            if(0 == strncmp(symbol->name,"_GLOBAL_OFFSET_TABLE_",21)) {
+                                symbol->addr = oc->info->got_start;
+                            } else {
+                                errorBelch("Failed to lookup symbol: %s\n",
+                                           symbol->name);
+                                return EXIT_FAILURE;
+                            }
                         }
                     } else {
                         // we already have the address.