BUG/MAJOR: fix build on musl with cpu_set_t support

BUG/MAJOR: fix build on musl with cpu_set_t support

Move cpu_map structure outside of the global struct to a global
variable defined in cpuset.c compilation unit. This allows to reorganize
the includes without having to define _GNU_SOURCE everywhere for the
support of the cpu_set_t.

This fixes the compilation with musl libc, most notably used for the
alpine based docker image.

This fixes the github issue #1235.

No need to backport as this feature is new in the current
2.4-dev.