Cleanup: __libc_init_AT_SECURE, auxv, sysinfo

__sanitize_environment_variables is only called when getauxval(AT_SECURE)
is true.

Instead of scanning __libc_auxv, reuse getauxval. If the entry is missing,
getauxval will set errno to ENOENT.

Reduce the number of times that __libc_sysinfo and __libc_auxv are
initialized. (Previously, __libc_sysinfo was initialized 3 times for the
linker's copy). The two variables are initialized in these places:
 - __libc_init_main_thread for libc.a (including the linker copy)
 - __libc_preinit_impl for libc.so
 - __linker_init: the linker's copy of __libc_sysinfo is still initialized
   twice, because __libc_init_main_thread runs after relocation. A later
   CL consolidates the linker's two initializations.

Bug: none
Test: bionic unit tests
Change-Id: I196f4c9011b0d803ee85c07afb415fcb146f4d65