pm: cpupower: Makefile: Fix cross compilation

pm: cpupower: Makefile: Fix cross compilation

After commit f79473ed9220 ("pm: cpupower: Makefile: Allow overriding
cross-compiling env params") we would fail to cross compile cpupower in
buildroot which uses the recipe at [1] where only the CROSS variable is
being set.

The issue here is the use of the lazy evaluation for all variables: CC,
LD, AR, STRIP, RANLIB, rather than just CROSS.

[1]:
https://git.buildroot.net/buildroot/tree/package/linux-tools/linux-tool-cpupower.mk.in

Fixes: f79473ed9220 ("pm: cpupower: Makefile: Allow overriding cross-compiling env params")
Reported-by: Florian Fainelli <florian.fainelli@broadcom.com>
Closes: https://lore.kernel.org/all/2bbabd2c-24ef-493c-a199-594e5dada3da@broadcom.com/
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>