[ELF] Refactor per-target TLS layout configuration. NFC.

Summary:
There are really three different kinds of TLS layouts:

 * A fixed TLS-to-TP offset. On architectures like PowerPC, MIPS, and
   RISC-V, the thread pointer points to a fixed offset from the start
   of the executable's TLS segment. The offset is 0x7000 for PowerPC
   and MIPS, which allows a signed 16-bit offset to reach 0x1000 of
   per-thread implementation data and 0xf000 of the application's TLS
   segment. The size and layout of the TCB isn't relevant to the static
   linker and might not be known.

 * A fixed TCB size. This is the format documented as "variant 1" in
   Ulrich Drepper's TLS spec. The thread pointer points to a 2-word TCB
   followed by the executable's TLS segment. The first word is always
   the DTV pointer. Used on ARM. The thread pointer must be aligned to
   the TLS segment's alignment, possibly creating alignment padding.

 * Variant 2. This format predates variant 1 and is also documented in
   Drepper's TLS spec. It allocates the executable's TLS segment before
   the thread pointer, apparently for backwards-compatibility. It's
   used on x86 and SPARC.

Factor out an lld::elf::getTlsTpOffset() function for use in a
follow-up patch for Android. The TcbSize/TlsTpOffset fields are only used
in getTlsTpOffset, so replace them with a switch on Config->EMachine.

Reviewers: espindola, ruiu, PkmX, jrtc27

Reviewed By: ruiu, PkmX, jrtc27

Subscribers: jyknight, emaste, sdardis, nemanjai, javed.absar, arichardson, kristof.beyls, kbarton, fedor.sergeev, atanasyan, PkmX, jsji, llvm-commits

Differential Revision: https://reviews.llvm.org/D53905

llvm-svn: 345775