linux-wandboard

[XTENSA] Add support for the sa_restorer function

Supporting the sa_restorer function allows for better security
since the sigreturn system call doesn't need to be placed on
the stack, so the stack doesn't need to be executable. This
requires support from the c-library as it has to provide the
restorer function.

Signed-off-by: Chris Zankel <chris@zankel.net>