cmd/5l, cmd/ld: add support for R_ARM_GOT_PREL

Android NDK's gcc 4.6 generates this relocation for runtime/cgo.

R=rsc
CC=golang-dev
https://golang.org/cl/6450056

src/cmd/5l/asm.c

@@ -158,11 +158,22 @@ adddynrel(Sym *s, Reloc *r)
 		r->sym = S;
 		r->add += targ->got;
 		return;
-	
+
+	case 256 + R_ARM_GOT_PREL: // GOT(S) + A - P
+		if(targ->dynimpname == nil || targ->dynexport) {
+			addgotsyminternal(targ);
+		} else {
+			addgotsym(targ);
+		}
+		r->type = D_PCREL;
+		r->sym = lookup(".got", 0);
+		r->add += targ->got + 4;
+		return;
+
 	case 256 + R_ARM_GOTOFF: // R_ARM_GOTOFF32
 		r->type = D_GOTOFF;
 		return;
-	
+
 	case 256 + R_ARM_GOTPC: // R_ARM_BASE_PREL
 		r->type = D_PCREL;
 		r->sym = lookup(".got", 0);

src/cmd/ld/elf.h

@@ -564,6 +564,7 @@ typedef struct {
 #define	R_ARM_PLT32		27	/* Add PC-relative PLT offset. */
 #define	R_ARM_CALL		28
 #define	R_ARM_V4BX		40
+#define	R_ARM_GOT_PREL		96
 #define	R_ARM_GNU_VTENTRY	100
 #define	R_ARM_GNU_VTINHERIT	101
 #define	R_ARM_RSBREL32		250

src/cmd/ld/ldelf.c

@@ -847,6 +847,7 @@ reltype(char *pn, int elftype, uchar *siz)
 	case R('5', R_ARM_REL32):
 	case R('5', R_ARM_CALL):
 	case R('5', R_ARM_V4BX):
+	case R('5', R_ARM_GOT_PREL):
 	case R('6', R_X86_64_PC32):
 	case R('6', R_X86_64_PLT32):
 	case R('6', R_X86_64_GOTPCREL):