build: add gcc sysroot for cross builds

Cross-builds of shared objects need a libc to link against (even though it
won't be used at runtime) and startup files.  Provide them in the gcc.bin
external repository for cross builds.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Update gcc.bin for glibc sysroot

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>