[compiler-rt] Don't use __libc_stack_end on ARM
Summary:
Commit
b684c1a50f70 ("Add a `Symbolizer::GetEnvP()` method that allows
symbolizer implementations to customise the environment of the
symbolizer binary.") exposed a latent ARM issue, and that broke
http://lab.llvm.org:8011/builders/clang-cmake-thumbv7-full-sh
This coincided with breakage caused by my commit
5f5fb56c68e4
("[compiler-rt] Intercept the uname() function"), so I had to
investigate.
The issue is that GetArgsAndEnv does not work on ARM: there glibc's
_start overwrites argc value stored at __libc_start_end, breaking the
existing argv/envp parsing logic.
Fix by inferring argc from argv.
Reviewers: eugenis, vitalybuka
Reviewed By: eugenis
Subscribers: dberris, kristof.beyls, danielkiss, #sanitizers, delcypher
Tags: #sanitizers
Differential Revision: https://reviews.llvm.org/D77400