elf/rtld: Count skipped environment variables for enable_secure

When using the glibc.rtld.enable_secure tunable we need to keep track of
the count of environment variables we skip due to __libc_enable_secure
being set and adjust the auxv section of the stack.  This fixes an
assertion when running ld.so directly with glibc.rtld.enable_secure set.
Add a testcase that ensures the assert is not hit.

elf/rtld.c:1324   assert (auxv == sp + 1);

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>