libsigsegv: disable for sparc and uClibc toolchain

The context function support is not available for uClibc and
sparc. It will be introduced in uClibc-ng 1.0.15, and
then the patch can be reverted.

Fixes following autobuild failures:
http://autobuild.buildroot.net/results/f1aa74992a77da325b3c6d74f9f88624bad094e9/
http://autobuild.buildroot.net/results/69bf0dbb0be3d4e4783ee332b3e95ca31f18d3bf/
http://autobuild.buildroot.net/results/f5bc9921c1b9f839e5536c19348efebd6826123d/



Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>