configure.ac: Bump the size of sigaltstack

The RubyVM uses C macro defines to feature detect whether
`backtrace(2)` support is available, and if so it includes C level backtraces
when the RubyVM itself crashes.

But on my machine, C level backtraces from `vm_dump.c` didn't work when
using a version of Ruby buillt on the machine, but worked fine when using a
version of Ruby built on another machine and copied to my machine.

The default autoconf test for backtraces uses a sigaltstack size that is
too small, so the SIGSEGV signal handler itself causes a SIGSEGV).
I noticed that signal.c uses a larger sigaltstack size:

https://github.com/ruby/ruby/blob/v2_6_5/signal.c#L568

The specific variables it looks at:

- `HAVE_BACKTRACE`

  this is a macro defined by autoconf because there is a line in the
  configure script like `AC_CHECK_FUNCS(backtrace)` (see the autoconf
  docs for more).

- `BROKEN_BACKTRACE`

  this comes from a custom program that Ruby's configure script runs to
  attempt to figure out whether actually using backtrace(2) in a real
  program works. You can see the autoconf program here.

  <https://github.com/ruby/ruby/blob/v2_6_5/configure.ac#L2817-L2863>

It uses sigaltstack and SA_ONSTACK to create a seperate stack for
handling signals.

The problem was: SIGSTKSZ (which comes from a system header!) was not
suggesting a large enough stack size. When checking on an Ubuntu 16.04
box, we found that SIGSTKSZ was 8192 and MINSIGSTKSZ was 2048.

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