rtasm: Generalize executable memory allocator to all Unices.
rtasm: Generalize executable memory allocator to all Unices.

We're only using fairly portable standard Unix calls here, so might as
well save ourselves future trouble by enabling on all Unices by default.

https://bugs.freedesktop.org/show_bug.cgi?id=90904

Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>
Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler