Fully support constructors for the purpose of code coverage analysis.
This involves:
1.  Have the loader pass the start and size of the .ctors section to the
    kernel in 2 new metadata elements.
2.  Have the linker backends look for and record the start and size of
    the .ctors section in dynamically loaded modules.
3.  Have the linker backends call the constructors as part of the final
    work of initializing preloaded or dynamically loaded modules.

Note that LLVM appends the priority of the constructors to the name of
the .ctors section. Not so when compiling with GCC. The code currently
works for GCC and not for LLVM.

Submitted by:	Dmitry Mikulin <dmitrym@juniper.net>
Obtained from:	Juniper Networks, Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273334 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f