[m-rev.] asm_fast.gc on MacOS

[Zoltan Somogyi]

I just added map_foldl4 to map.m and tree234.m. In the course of
adding a mention to NEWS, I came across this:

* The asm_fast* and reg* grades now work on 64-bit Intel OS X systems when
  using GCC as the C compiler.
  See README.MacOS for further details.

This was true when that paragraph was added in 2014, but I believe
it hasn't been true for a while now, due to newer gcc versions.

Could someone who knows the full story please update both this
paragraph and README.MacOS?

Zoltan.