nptl/headers: Convert the makefile targets into variables.

nptl/headers: Convert the makefile targets into variables.

* Introduce pregen-headers-y which is list of generated headers
  during the build and add this to pregen target in top level makefile.
  Also add nptl_headers, nptl_arch_headers, nptl_linux_headers
  to this if compiling with nptl.

* Replace pthread_pregen var with pregen-headers-y.

* Make nptl_headers, nptl_arch_headers, nptl_linux_headers variables
  instead of sub targets.

Signed-off-by: Khem Raj <raj.khem@gmail.com>