extend gthr-posix.h with rwlock

[David Edelsohn]

>>>>> Luke Dalessandro writes:

Luke> My problem is that unwind-dw2-fde.c seems to be compiled multiple times during 
Luke> a gcc build, and sometimes my additions are found but other times they are 
Luke> not. I am rebuilding again (AIX 5.1), and I'll post more information for 
Luke> anyone that needs it.

Luke> In the meantime, is there a how-to anywhere that describes adding or modifying 
Luke> gthr.h models in gcc?

	AIX multilibs pthread support.  Unlike Linux, AIX does not provide
weak versions of the pthread symbols when operating in single-threaded
mode.  AIX uses gthr-aix.h, which includes gthr-posix.h or gthr-single.h
depending on the -pthread option.

David