For cross-builds rely on `_POSIX_C_SOURCE` to decide if `poll()` is
supported, rather than just assuming it isn't.
This may still miss to detect `poll()` support, as seen for example with
Linux MUSL cross-builds.
Also:
- GHA/curl-for-win: enable RISC-V 64 cross-target for Linux MUSL.
(to test this case with cmake, with a false-negative.)
The first RISC-V 64 build in curl's CI.
- GHA/curl-for-win: add arm64/intel64 job for Linux glibc.
(to test this case with cmake, and succeed.)
- cmake: delete unnecessary `#include <sys/time.h>` from non-cross-build
`poll()` detection snippet.
Follow-up tp cc8b813765#14718Fixes#14714Closes#14734
04e3621dce