tools/power/cpupower: Support building libcpupower statically

tools/power/cpupower: Support building libcpupower statically

The cpupower Makefile built and installed libcpupower as a shared
library (libcpupower.so) without passing `STATIC=true`, but did not
build a static version of the library even with `STATIC=true`. (Only the
programs were static). Thus, out-of-tree programs using libcpupower
were unable to link statically against the library without having access
to intermediate object files produced during the build.

This fixes that situation by ensuring that libcpupower.a is built and
installed when `STATIC=true` is specified.

Link: https://lore.kernel.org/r/x7geegquiks3zndiavw2arihdc2rk7e2dx3lk7yxkewqii6zpg@tzjijqxyzwmu
Signed-off-by: Zuo An <zuoan.penguin@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>