ELF/AArch64: Fix dynamic relocation against local symbol in shared objects

AArch64 TLSDESC for local symbol in shared objects are implemented in a
arch specific manner where the TLSDESC dynamic relocation addend is the
symbol VM inside the TLS block. For instance, with a shared library
created from the code:

--
static __thread int32_t x1;
static __thread int64_t x2;

int32_t foo1 (int32_t x)
{
  x1 += x;
  return x;
}

int64_t foo2 (int64_t x)
{
  x2 += x;
  return x;
}
--

The dynamic relocation should be create as:

Relocations [
  Section (N) .rela.dyn {
    <Address1> R_AARCH64_TLSDESC - 0x0
    <Address2> R_AARCH64_TLSDESC - 0x8
  }
]

Where 0x0 addend in first dynamic relocation is the address of 'x1'
in TLS block and '0x8' is the address of 'x2'.

Checked against test-suite on aarch64-linux-gnu.

llvm-svn: 290099