Move BFD_FAKE_SECTION to libbfd.h

Move BFD_FAKE_SECTION to libbfd.h

BFD_FAKE_SECTION and its sidekick GLOBAL_SYM_INIT don't need to be
cluttering bfd.h, and probably shouldn't be used outside bfd/.  To
make them internal to bfd, make the bfd ecoff small common section
declaration global so it can be used instead of a duplicate in
gas/ecoff.c.  Oddly this needs to go in bfd/ecofflink.c rather than
bfd/ecoff.c as the former is compiled for all targets needing the
ecoff small common section (some via a call in gas/config/obj-elf.c to
a function in gas/ecoff.c) while the latter is not.

While doing this rename ecoff_scom_section to _bfd_ecoff_scom_section
and remove support for traditional C from GLOBAL_SYM_INIT.