target/8336: bootstrap config still tries to use COFF options

[David Sullivan]

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8
336

I was a bit foggy when I submitted this this morning.  Apologies to all for
that.  The issue is within the Makefile in the "gcc" subdirectory.  When you
run "./configure --with-gnu-as", it places a "-mcoff" option in the makefile
variable CRTSTUFF_T_CFLAGS_S.  Without the "--with-gnu-as", this variable isn't
present in the makefile at all.  My assumption was that the problem was in the
file "sco5.h".