libsigsegv: add RISC-V support

This commit backports an upstream commit adding RISC-V support to
libsigsegv, and adds a patch that finalizes the RISC-V support to make
it compatible with cross-compilation.

Fixes:

  http://autobuild.buildroot.net/results/bd12cb3db3b46860a4ab8fb91052319dce75202a/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Mark Corbin <mark.corbin@embecosm.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>