config: move the "build shared libraries" option to the OS menu

The "Build shared libraries" config option is dependant on the type of
"Target OS".

Moving this options to the "Target OS" sub-menu is also better in the user
perspective: he/she no longer needs to go back and forth to see if he/she
missed any option.

 # ia64 specific config options

 # depends on EXPERIMENTAL

 config ARCH_ia64

     select ARCH_USE_MMU

     select ARCH_64

     select ARCH_SUPPORTS_BOTH_ENDIAN

     help

       The ia64 architecture, as defined by:

         http://www.intel.com/design/itanium/arch_spec.htm