Currently, GCC accepts --with-sysroot without an argument, whereas ld
and gdb don't. Also, GCC's default sysroot is not considered
relocatable, even though the whole point of picking that default was
such that it was.  This patch arranges for all three tools to use the
same relocatable default.  Ok to install in ld and gdb?  (I'll install
it in gcc when I get approval for the others).