rtld: introduce STATIC_TLS_EXTRA

The new STATIC_TLS_EXTRA variable provides a means for applications
to increases the size of the extra static TLS space allocated by
rtld beyond the default of '128'. This extra static TLS space is used
for objects loaded with dlopen.

The value specified in the variable must be no less than the default
value and no greater than the maximum allowed value for size_t type.

If an invalid value is specified, rtld will ignore it and just use
the default value.

The rtld(1) man page is updated to document this new option.

Obtained from: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D42025