ELF/ARM: Ignore R_ARM_V4BX for ARMv4 but allow linking

This patch allow the ARM relocation R_ARM_V4BX to be processed by lld,
although it is not really handled in the static relocation code.  The
relocation is in the form:

Relocation section '.rel.text' at offset 0x428 contains 4 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
00000014  00000028 R_ARM_V4BX

Meaning it does have a direct target, but rather references to an absolute
section *ABS* (in this exemple to the .text segment itself).  It makes the
target Atom after file parse to not have a associated pointer and thus
generating a derrefence NULL point in ELFFile<ELFT>::findAtom.  Current
approach is just ignore and return nullptr in such cases.

The problem relies that default GCC configuration
for arm-linux-gnueabi{hf} emits the relocation for the asm:

--
.syntax unified
.arm

.p2align 2
.type fn, %function
fn:
  ldr r3, .LGOT
  ldr r2, .LGOT+4
.LPIC:
  add r3, pc, r3
  ldr r2, [r3, r2]
  cmp r2, #0
  bxeq lr
  b __start__
.LGOT:
 .word _GLOBAL_OFFSET_TABLE_-(.LPIC+8)
 .word __start__(GOT)
--

But only with the option -march=armv4 (which is the default GCC configuration).
For arm5 and forward the relocation is not created. This a special relocation
(defined miscellaneous for ARM) that instruct the linker to replace the bx
instruction into a mov.  GNU linker has some options related to which substitution
it can create for such cases.

With this patch I can dynamically link an application against a GLIBC
arm-linux-gnueabi system configured with default GCC.

llvm-svn: 235880