[XRay][CodeGen] Use PIC-friendly code in XRay sleds; remove synthetic references in .text

Summary:
This change achieves two things:

  - Redefine the Custom Event handling instrumentation points emitted by
    the compiler to not require dynamic relocation of references to the
    __xray_CustomEvent trampoline.

  - Remove the synthetic reference we emit at the end of a function that
    we used to keep auxiliary sections alive in favour of SHF_LINK_ORDER
    associated with the section where the function is defined.

To achieve the custom event handling change, we've had to introduce the
concept of sled versioning -- this will need to be supported by the
runtime to allow us to understand how to turn on/off the new version of
the custom event handling sleds. That change has to land first before we
change the way we write the sleds.

To remove the synthetic reference, we rely on a relatively new linker
feature that preserves the sections that are associated with each other.
This allows us to limit the effects on the .text section of ELF
binaries.

Because we're still using absolute references that are resolved at
runtime for the instrumentation map (and function index) maps, we mark
these sections write-able. In the future we can re-define the entries in
the map to use relative relocations instead that can be statically
determined by the linker. That change will be a bit more invasive so we
defer this for later.

Depends on D36816.

Reviewers: dblaikie, echristo, pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36615

llvm-svn: 311525