Hi all, this patch does two things:

(A) Documentation for -fopenmp

This finally adds documentation - which was lacking a long time but
is increasingly demanded. Often used options:
  -foffload='-lm' and '-lgfortran'
  -foffload=(nvptx-none=)-latomic
  -foffload=(amdgcn-amdhsa=)-march=gfx906

  * * *

(B) -foffload=default

Came up for the testsuite as libatomic is sometimes needed for nvptx
and unsupported (not build) for GCN.
When GCC is configured to support more than one offload target:
* -foffload=-latomic →  fails to build on GCN
* -foffload=nvptx-none=-latomic →  disables all but nvptx

However, it might be also useful in the real world.

  * * *

Loosely tested without offloading support and with only one offload target.
As the following works, I assume the patch is fine – but I would not mind
if someone else would test it, who built GCC with multiple offload targets.

For testing, I want to note that
   gcc -v 2>&1|grep OFFLOAD_TARGET
shows the configured targets (and OFFLOAD_TARGET_DEFAULT=1 when
configured with --enable-offload-defaulted).

As soon as -foffload=... has been specified, the output is changed to
only include the specified devices.

  * * *

Thoughts? Comments? Suggestions? Test results? OK?

  * * *

Crossref:

* Configure option --enable-offload-defaulted,
   see https://gcc.gnu.org/install/configure.html
* PR about documenting -foffload: https://gcc.gnu.org/PR67300
* Current documentation in the wiki: https://gcc.gnu.org/wiki/Offloading
   (First line: 'compilation options' link)
* Email thread about -foffload=default, -foffload=nvptx-none=-latomic
   disabling other hosts and (next in thread) automatically linking
   -latomic.
   https://gcc.gnu.org/pipermail/gcc-patches/2021-May/570627.html

Tobias

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