SenoraGC: support allocation of executable memory; tune for performance

Allocation of executable memory is intended to make SELinux
happier by mmapping with PROT_EXEC instead of using mprotect()
to allow execution after the fact.

Performance improvements bring SGC within 30% of the Boehm GC on
`racketcgc -cl racket`, which makes SGC an even more plausible
substitute.
racketcgc: use SenoraGC instead of Boehm GC by default

This new default for Unix and Mac OS X trades performance for
portability (hopefully), but for most users the switch affects only
for the build process, where `racketcgc` is used to build `racket`.

To continue using Boehm GC, configure with `--disable-sgc`.

For now, Boehm GC continues to be the default for Windows.