eventfd: allow for all CPUs

After fixing support for x32, unlock eventfd support for all CPUs.
Before this patch, it was explicitly limited to 64-bit ones.

You can disable eventfs manually on systems where it's auto-detected:
- cmake: `-DHAVE_EVENTFD=0`
- configure: `export ac_cv_func_eventfd=0`

Ref: c2aa504ab9 #16239
Closes #16277

lib/socketpair.h

@@ -26,24 +26,7 @@
 
 #include "curl_setup.h"
 
-#if defined(HAVE_EVENTFD) && \
-    (defined(__x86_64__) || \
-     defined(__aarch64__) || \
-     defined(__ia64__) || \
-     defined(__ppc64__) || \
-     defined(__mips64) || \
-     defined(__sparc64__) || \
-     defined(__riscv_64e) || \
-     defined(__s390x__))
-
-/* Use eventfd only with 64-bit CPU architectures because eventfd has a
- * stringent rule of requiring the 8-byte buffer when calling read(2) and
- * write(2) on it. In some rare cases, the C standard library implementation
- * on a 32-bit system might choose to define uint64_t as a 32-bit type for
- * various reasons (memory limitations, compatibility with older code),
- * which makes eventfd broken.
- */
-#define USE_EVENTFD 1
+#ifdef HAVE_EVENTFD
 
 #define wakeup_write  write
 #define wakeup_read   read
@@ -63,7 +46,7 @@ int Curl_eventfd(curl_socket_t socks[2], bool nonblocking);
 #include <curl/curl.h>
 int Curl_pipe(curl_socket_t socks[2], bool nonblocking);
 
-#else /* !USE_EVENTFD && !HAVE_PIPE */
+#else /* !HAVE_EVENTFD && !HAVE_PIPE */
 
 #define wakeup_write     swrite
 #define wakeup_read      sread
@@ -86,7 +69,7 @@ int Curl_pipe(curl_socket_t socks[2], bool nonblocking);
 #define wakeup_create(p,nb)\
 Curl_socketpair(SOCKETPAIR_FAMILY, SOCKETPAIR_TYPE, 0, p, nb)
 
-#endif /* USE_EVENTFD */
+#endif /* HAVE_EVENTFD */
 
 #ifndef CURL_DISABLE_SOCKETPAIR
 #include <curl/curl.h>