Hi,

This patch considers the environment variable syntax extension for
device-specific variants of environment variables from OpenMP 5.1 (see
OpenMP 5.1 specification, p. 75 and p. 639). An environment variable
(e.g. OMP_NUM_TEAMS) can have different suffixes:

_DEV (e.g. OMP_NUM_TEAMS_DEV): affects all devices but not the host.
_DEV_<device> (e.g. OMP_NUM_TEAMS_DEV_42): affects only device with
number <device>.
no suffix (e.g. OMP_NUM_TEAMS): affects only the host.

In future OpenMP versions also suffix _ALL will be introduced (see
discussion https://github.com/OpenMP/spec/issues/3179). This is also
considered in this patch:

_ALL (e.g. OMP_NUM_TEAMS_ALL): affects all devices and the host.

The precedence is as follows (descending). For the host:

     1. no suffix
     2. _ALL

For devices:

     1. _DEV_<device>
     2. _DEV
     3. _ALL

That means, _DEV_<device> is used whenever available. Otherwise _DEV is
used if available, and at last _ALL. If there is no value for any of the
variable variants, default values are used as already implemented before.

This patch concerns parsing (a), storing (b), output (c) and
transmission to the device (d):

(a) The actual number of devices and the numbering are not known when
parsing the environment variables. Thus all environment variables are
iterated and searched for device-specific ones.

(b) Only configured device-specific variables are stored. Thus, linked
lists are used.

(c) The output is done in omp_display_env (see specification p. 468f).
Global ICVs are tagged with [all], see
https://github.com/OpenMP/spec/issues/3179. ICVs which are not global
but aren't handled device-specific yet are tagged with [host].
omp_display_env outputs the initial values of the ICVs. That's why
separate data structures are introduced (like gomp_initial_icv...).

(d) Device-specific ICVs which are already user accessible on the device
are transmitted to the device (moreover nteams-var is added and used for
the tests). There are ICVs which values are currently set explicitly in
the config when copying them to the device: GOMP_NTHREADS_VAR,
GOMP_THREAD_LIMIT_VAR, GOMP_DYN_VAR (see gomp_gcn_enter_kernel in
libgomp/config/gcn/team.c and gomp_nvptx_main in
libgomp/config/nvptx/team.c). The corresponding environment variables
are nevertheless parsed and stored device-specific but the transmission
to the device is not changed.

Marcel
-----------------
Siemens Electronic Design Automation GmbH; Anschrift: Arnulfstraße 201, 80634 München; Gesellschaft mit beschränkter Haftung; Geschäftsführer: Thomas Heurung, Frank Thürauf; Sitz der Gesellschaft: München; Registergericht München, HRB 106955