[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

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