[SYCL][Doc] Modernize GroupMask extension

[Pennycook]

Brings GroupMask extension in line with other extensions:

• Written against SYCL 2020, with appropriate function names
• Feature test macro
• sycl::ext::oneapi namespace

This commit replaces the old SYCL 1.2.1 extension completely
in favor of a SYCL 2020 extension, because the SYCL 1.2.1 extension
was never implemented.

Signed-off-by: John Pennycook [email protected]

[Pennycook]

Opened as a draft because AsciiDoc does not want to format the +, ^ or | characters used to describe operators.

@mkinsner, @rolandschulz, @jbrodman, @gmlueck -- is this something you've encountered with any other extensions? I tried using the +++ passthrough syntax but it just made things worse.

[keryell]

There is some shift in the table after row containing group_mask operator &=(group_mask rhs), probably related to an error syntax because of magic role of + as you mention.

[Pennycook]

There is some shift in the table after row containing group_mask operator &=(group_mask rhs), probably related to an error syntax because of magic role of + as you mention.

Yes! AsciiDoc is infuriating! But as luck would have it, I think I just fixed everything. Would you mind taking a quick second look?

An explanation of the changes, and a note to myself for future reference:

• pass:[...] is the inline passthrough syntax respected by AsciiDoc to stop interpretation of special characters (e.g. pass:[+])
• The passthrough syntax doesn't work for column separators in tables, but a regular escape does (i.e. \|)