This is simplified version of patch originally proposed in 
https://sourceware.org/ml/binutils/2014-03/msg00046.html

This code will add pretty-printing of PLT entries on ARM, so e.g. instead of

  Disassembly of section .plt:

  00001000 <.plt>:
     1000:	e52de004 	push	{lr}		; (str lr, [sp, #-4]!)
     1004:	e59fe004 	ldr	lr, [pc, #4]	; 1010 <_start-0x14>
     1008:	e08fe00e 	add	lr, pc, lr
     100c:	e5bef008 	ldr	pc, [lr, #8]!
     1010:	f00000f0 	.word	0xf00000f0
     1014:	e28fc20f 	add	ip, pc, #-268435456	; 0xf0000000
     1018:	e28cc600 	add	ip, ip, #0, 12
     101c:	e28cca00 	add	ip, ip, #0, 20
     1020:	e5bcf0f0 	ldr	pc, [ip, #240]!	; 0xf0

objdump will now output a much more convenient message:

  Disassembly of section .plt:

  00001000 <foo@plt-0x14>:
     1000:	e52de004 	push	{lr}		; (str lr, [sp, #-4]!)
     1004:	e59fe004 	ldr	lr, [pc, #4]	; 1010 <foo@plt-0x4>
     1008:	e08fe00e 	add	lr, pc, lr
     100c:	e5bef008 	ldr	pc, [lr, #8]!
     1010:	f00000f0 	.word	0xf00000f0

  00001014 <foo@plt>:
     1014:	e28fc20f 	add	ip, pc, #-268435456	; 0xf0000000
     1018:	e28cc600 	add	ip, ip, #0, 12
     101c:	e28cca00 	add	ip, ip, #0, 20
     1020:	e5bcf0f0 	ldr	pc, [ip, #240]!	; 0xf0

Instead of abusing elf_backend_plt_sym_val I overloaded 
bfd_elf32_get_synthetic_symtab (per Pavel's advice). This has 
significantly shortened the patch.

I got no regressions for host-only tests on 
arm-{gnueabi,eabi,nacl,symbian}. I haven't tried runtime gdb tests (they 
require complete toolchains for all platforms and I don't have them atm).

Ok to commit?

-Y