x86, vdso: Don't quote $nm in the script for checking vdso references

Don't quote $nm in the script for checking the vdso for external
references.  Doing so breaks multiword constructs, like using
CROSS_COMPILE='ccache '.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
LKML-Reference: <20100728134252.2e4c27cf.sfr@canb.auug.org.au>