vxworks: enable use of .init_array/.fini_array for cdtors

vxworks: enable use of .init_array/.fini_array for cdtors

The target OS actually runs all function pointers found in the _ctors
array when the module is loaded. So it is not that hard to make use of
the "modern" .init_array/.fini_array mechanism - it mostly just requires
a linker script adding the _ctors and _dtors symbols and terminating
LONG(0) entries.

Assume that if the user passed --enable-initfini-array when building
gcc, the rest of the toolchain (including the link spec and linker
script) is set up appropriately.

Note that configuring with --enable-initfini-array may prevent the -mrtp
mode from working, due to the (unconditional) use of .init_array.*
sections instead of .ctors.* - however, that is the case regardless of this
patch.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263691 138bc75d-0d04-0410-961f-82ee72b054a4