shared libgcc build support

[David Edelsohn]

	The patch does allow me to create AIX shared libraries, although
the SHLIB_LINK line is pretty horrific:

# Build a shared libgcc library.
SHLIB_EXT = .a
SHLIB_LINK = $(GCC_FOR_TARGET) $(LIBGCC2_CFLAGS) -shared -nodefaultlibs \
        -Wl,-bexpall -o shr.o @multilib_flags@ @shlib_objs@ -lc \
        `case @shlib_base_name@ in \
        *pthread*) echo -L/usr/lib/threads -lpthreads -lc_r /usr/lib/libc.a ;; \
        *) echo -lc ;; esac` ; \
        rm -f tmp-@shlib_base_name@.a ; \
        $(AR_CREATE_FOR_TARGET) tmp-@shlib_base_name@.a shr.o ; \
        mv tmp-@shlib_base_name@.a @shlib_base_name@.a ; \
        rm -f shr.o

	Also, are you planning to provide the library interface version
number as a substitution variable instead of a fixed value?  shr.o above
really should be shr0.o or shr_gcc3.o or shr_libgcc0.o or something with a
version number embedded in the member name.

Thanks, David