external-sourcery-toolchain: Error out if libc.so does not exist

Depending on the external toolchain config, the libc.so file may
not exist.   This generally means that the machine config does
not have multi-lib support when it should.  Rather than letting
the "sed" command error out with a cryptic message, issue a
(hopefully) more meaningful message.

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>