[X86] Fix x86-64 call *foo@tlsdesc(%rax) and support R_386_TLSGOTDESC R_386_TLS_DESC_CALL

D18885 emitted 5 bytes for call *foo@tlsdesc(%rax). It should use the
2-byte form instead and let R_X86_64_TLSDESC_CALL apply to the beginning
of the call instruction.

The 2-byte form was deliberately chosen to make ->LE and ->IE relaxation work:

    0:   48 8d 05 00 00 00 00    lea    0x0(%rip),%rax        # 7 <.text+0x7>
                         3: R_X86_64_GOTPC32_TLSDESC     a-0x4
    7:   ff 10                   callq  *(%rax)
                         7: R_X86_64_TLSDESC_CALL        a

=>

    0:   48 c7 c0 fc ff ff ff    mov    $0xfffffffffffffffc,%rax
    7:   66 90                   xchg   %ax,%ax

Also change the symbol type to STT_TLS when VK_TLSCALL or VK_TLSDESC is
seen.

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D62512

llvm-svn: 361910