c-stack: work around Solaris 11 bugs

c-stack: work around Solaris 11 bugs

Problem reported by Bruno Haible in:
https://lists.gnu.org/r/bug-gnulib/2021-05/msg00062.html
* lib/c-stack.c: Always include sigsegv.h if HAVE_LIBSIGSEGV.
(USE_LIBSIGSEGV): Do not use libsigsegv if the kernel
has the si_addr bug and libsigsegv is too old to work
around it.
(segv_handler) [!USE_LIBSIGSEGV]: Do not trust si_addr
if BOGUS_SI_ADDR_UPON_STACK_OVERFLOW.
* m4/c-stack.m4 (AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC):
Define BOGUS_SI_ADDR_UPON_STACK_OVERFLOW on Solaris 2.11 SPARC.
And do not define HAVE_XSI_STACK_OVERFLOW_HEURISTIC.