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

arch/x86/vdso/checkundef.sh

 #!/bin/sh
 nm="$1"
 file="$2"
-"$nm" "$file" | grep '^ *U' > /dev/null 2>&1
+$nm "$file" | grep '^ *U' > /dev/null 2>&1
 if [ $? -eq 1 ]; then
     exit 0
 else