Fix NetBSD build issue: Use compiler version of alloca(3) when possible

Fix NetBSD build issue: Use compiler version of alloca(3) when possible

The documentation of alloca(3) says:

     o   The alloca() function should be supplied by the compiler because the
         compiler is allowed to make assumptions about the stack and frame
         pointers.  The libc alloca() implementation cannot account for those
         assumptions.  While there is a machine dependent implementation of
         alloca() in libc, its use is discouraged and in most cases it will
         not work.  Using this implementation will produce linker warnings.

This is true for CoreCLR - alloca(3) was breaking 30 PAL tests.

Enforce on this operating system a compiler version.

Thanks Jan Kotas (Microsoft) for inspiration what may go wrong.
Thanks Jan Vorli (Microsoft) for useful suggestions in order to fix it.