unix: Auto-detect MICROPY_EMIT_X64 and MICROPY_GCREGS_SETJMP.

If not set, MICROPY_EMIT_X64 is set only if on x86-64 machine.

If not set, MICROPY_GCREGS_SETJMP is set when on MIPS.
Code style/whitespace cleanup; remove obsolete headers.

And move the MAP_ANON redefinition from py/asmx64.c to unix/alloc.c.
Basic nspire support
nsp module

-Texture class
-waitKeypress function
-sys.exit exits cleanly
-3MiB heap
Add nsp.readRTC() function and turn on LTO and -Ofast
Get rid of unnecessary __del__
Add greyscale support (needs modified nspire-io)
Add nsp.Texture.setData function
Add base64 decoding for nsp.Texture.setData
Add msgbox if heap alloc failed