[scudo] Workaround for uninitialized Bionic globals

Summary:
Bionic doesn't initialize its globals early enough. This causes issues when
trying to access them from a preinit_array (b/25751302) or from another
constructor called before the libc one (b/68046352). __progname is initialized
after the other globals, so we can check its value to know if calling
`getauxval` is safe.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: srhines, llvm-commits

Differential Revision: https://reviews.llvm.org/D40504

llvm-svn: 319099