Arm: Fix LSB of GOT for Thumb2 only PLT.

When you have a Thumb only PLT then the address in the GOT for PLT0 needs to
have the Thumb bit set since the instruction used in PLTn to get there is
`ldr.w	pc` which is an inter-working instruction:

the PLT sequence in question is

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>

and previously the linker would generate

Hex dump of section '.got':
 ...
  0x000101b8 40010100 00000000 00000000 10010000 @...............

Which would make it jump and transition out of thumb mode and crash since you
only have thumb mode on such cores.

Now it correctly generates

Hex dump of section '.got':
 ...
  0x000101b8 40010100 00000000 00000000 11010000 @...............

Thanks to Amol for testing patch and to rgujju for reporting it.

bfd/ChangeLog:

	PR ld/16017
	* elf32-arm.c (elf32_arm_populate_plt_entry): Set LSB of the PLT0
	address in the GOT if in thumb only mode.

ld/ChangeLog:

	PR ld/16017
	* testsuite/ld-arm/arm-elf.exp (thumb-plt-got): New.
	* testsuite/ld-arm/thumb-plt-got.d: New test.

1 files changed, 6 insertions, 0 deletions

diff --git a/bfd/elf32-arm.c b/bfd/elf32-arm.c
index 0036ff9..02d43a8 100644
--- a/bfd/elf32-arm.c
+++ b/bfd/elf32-arm.c
@@ -10001,6 +10001,12 @@ elf32_arm_populate_plt_entry (bfd *output_bfd, struct bfd_link_info *info,
 	      rel.r_info = ELF32_R_INFO (dynindx, R_ARM_JUMP_SLOT);
 	      initial_got_entry = (splt->output_section->vma
 				   + splt->output_offset);
+
+	      /* PR ld/16017
+		 When thumb only we need to set the LSB for any address that
+		 will be used with an interworking branch instruction.  */
+	      if (using_thumb_only (htab))
+		initial_got_entry |= 1;
 	    }
 	}