Arm: Fix thumb2 PLT branch offsets.

When I previously changed these offsets I had incorrectly used an offset of -2
for this Thumb2 PLT.  Unfortunately because we had no tests for this PLT I had
missed that the result was incorrect.

This patch fixes the offset to PC .-4 so that it correctly addresses the
previous instruction and adds a test for this PLT stub.

bfd/ChangeLog:

	* elf32-arm.c (elf32_thumb2_plt_entry): Fix PC-rel offset.

ld/ChangeLog:

	* testsuite/ld-arm/arm-elf.exp (thumb-plt): New.
	* testsuite/ld-arm/thumb-plt.d: New test.
	* testsuite/ld-arm/thumb-plt.s: New test.

bfd/ChangeLog

@@ -1,3 +1,7 @@
+2020-04-01  Tamar Christina  <tamar.christina@arm.com>
+
+	* elf32-arm.c (elf32_thumb2_plt_entry): Fix PC-rel offset.
+
 2020-04-01  Hans-Peter Nilsson  <hp@bitrange.com>
 
 	* mmo.c (mmo_scan): Create .text section only when needed, not

bfd/elf32-arm.c

@@ -2453,8 +2453,8 @@ static const bfd_vma elf32_thumb2_plt_entry [] =
   0x0c00f240,		/* movw	   ip, #0xNNNN	  */
   0x0c00f2c0,		/* movt	   ip, #0xNNNN	  */
   0xf8dc44fc,		/* add	   ip, pc	  */
-  0xe7fdf000		/* ldr.w   pc, [ip]	  */
-			/* b      .-2		  */
+  0xe7fcf000		/* ldr.w   pc, [ip]	  */
+			/* b      .-4		  */
 };
 
 /* The format of the first entry in the procedure linkage table

ld/ChangeLog

@@ -1,3 +1,9 @@
+2020-04-01  Tamar Christina  <tamar.christina@arm.com>
+
+	* testsuite/ld-arm/arm-elf.exp (thumb-plt): New.
+	* testsuite/ld-arm/thumb-plt.d: New test.
+	* testsuite/ld-arm/thumb-plt.s: New test.
+
 2020-04-01  Hans-Peter Nilsson  <hp@bitrange.com>
 
 	* testsuite/ld-scripts/defined4.d: Don't xfail mmix-*-*.

ld/testsuite/ld-arm/arm-elf.exp

@@ -1268,3 +1268,5 @@ run_dump_test "non-contiguous-arm3"
 run_dump_test "non-contiguous-arm4"
 run_dump_test "non-contiguous-arm5"
 run_dump_test "non-contiguous-arm6"
+
+run_dump_test "thumb-plt"

ld/testsuite/ld-arm/thumb-plt.d

@@ -0,0 +1,34 @@
+#source: thumb-plt.s
+#name: Thumb only PLT and GOT
+#ld: -shared -e0
+#objdump: -dr
+#skip: *-*-pe *-*-wince *-*-vxworks armeb-*-* *-*-gnueabihf
+
+.*: +file format .*arm.*
+
+
+Disassembly of section \.plt:
+
+00000110 <\.plt>:
+ 110:	b500      	push	{lr}
+ 112:	f8df e008 	ldr.w	lr, \[pc, #8\]	; 11c <\.plt\+0xc>
+ 116:	44fe      	add	lr, pc
+ 118:	f85e ff08 	ldr.w	pc, \[lr, #8\]!
+ 11c:	0001009c 	\.word	0x0001009c
+
+00000120 <foo@plt>:
+ 120:	f240 0c98 	movw	ip, #152	; 0x98
+ 124:	f2c0 0c01 	movt	ip, #1
+ 128:	44fc      	add	ip, pc
+ 12a:	f8dc f000 	ldr.w	pc, \[ip\]
+ 12e:	e7fc      	b.n	12a <foo@plt\+0xa>
+
+Disassembly of section .text:
+
+00000130 <bar>:
+ 130:	b580      	push	{r7, lr}
+ 132:	af00      	add	r7, sp, #0
+ 134:	f7ff fff4 	bl	120 <foo@plt>
+ 138:	4603      	mov	r3, r0
+ 13a:	4618      	mov	r0, r3
+ 13c:	bd80      	pop	{r7, pc}

ld/testsuite/ld-arm/thumb-plt.s

@@ -0,0 +1,18 @@
+	.cpu cortex-m3
+	.text
+	.align	1
+	.global	bar
+	.arch armv7-m
+	.syntax unified
+	.thumb
+	.thumb_func
+	.fpu softvfp
+	.type	bar, %function
+bar:
+	push	{r7, lr}
+	add	r7, sp, #0
+	bl	foo(PLT)
+	mov	r3, r0
+	mov	r0, r3
+	pop	{r7, pc}
+	.size	bar, .-bar