buildroot

uclibc: fix mips/mips64 build with older compilers

For example gcc 4.8.x fails to build uClibc-ng for mips/mips64.
Check if feature exist before using it.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>