libsigsegv: not available on SuperH with uClibc

Like PowerPC, the uClibc port on SuperH doesn't implement ucontext, so
we can't build libsigsegv in such a configuration. Therefore this
commit marks libsigsegv as not available on SuperH/uClibc.

Fixes:

  http://autobuild.buildroot.net/results/2128a3507e57e707ee5c726f1ccfddadb7b19112/



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