elf: Make glibc.rtld.enable_secure ignore alias environment variables

Tunable with environment variables aliases are also ignored if
glibc.rtld.enable_secure is enabled.  The tunable parsing is also
optimized a bit, where the loop that checks each environment variable
only checks for the tunables with aliases instead of all tables.

Checked on aarch64-linux-gnu and x86_64-linux-gnu.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>