rts: Non-portable pthread related code for NetBSD

Add NetBSD-specific code for non-portable parts of rts.

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to [Notes][notes] and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a [Note][notes] and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• replace this message with a description motivating your change