Rework Linux (and *BSD) interceptors to allow for up to 3 (2 for *BSD)
simultaneous interceptors. See code comments for details.
The main motivation is to support new sampling sanitizers (in the spirit
of GWP-ASan), that have to intercept few functions. Unfortunately, the
reality is that there are user interceptors that exist in the wild.
To support foreign user interceptors, foreign dynamic analysis
interceptors, and compiler-rt interceptors all at the same time,
including any combination of them, this change enables up to 3
interceptors on Linux (2 on *BSD).
v2:
* Revert to to the simpler "weak wrapper -(alias)-> __interceptor"
scheme on architectures that cannot implement a trampoline efficiently
due to complexities of resolving a preemptible symbol (PowerPC64
ELFv2 global entry, and i386 PIC).
* Avoid duplicate intercepted functions in gen_dynamic_list.py, due to
matching __interceptor_X and ___interceptor_X.
* Fix s390 __tls_get_offset.
Reviewed By: dvyukov, MaskRay, vitalybuka
Differential Revision: https://reviews.llvm.org/D151085
37445e96d8