clapack: don't allow on PowerPC uClibc

On PowerPC, clapack uses the _fpu_control symbol, which isn't defined
on uClibc, so this commit disallows the selection of clapack on
PowerPC uClibc configurations. PowerPC/glibc and ARM/uClibc
configurations have been tested to build correctly.

Fixes:

  http://autobuild.buildroot.org/results/d1d/d1d721fa697956218dfc2c865dfb61911cf2600e/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>