uclibc-ng/uclibc: enable context functions

For mongrel2/libsigsegv we need to enable this in the uClibc-ng/uClibc-snapshot config.
Tested with ARM:

file output/target/usr/bin/mongrel2
output/target/usr/bin/mongrel2: ELF 32-bit LSB executable, ARM, version 1 (SYSV), dynamically linked (uses shared libs), stripped

Should fix:
http://autobuild.buildroot.net/results/a144bc6024415a5272c3cbe60ff636d078d0a555/
http://autobuild.buildroot.net/results/00195d89a115a314bf4916af127407f61cd1b418/
http://autobuild.buildroot.net/results/5ee7e83ebcfa43f6ee0a2a8b9f40f1ae84664aca/



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