On riscv64 require libatomic if actually needed

clang and newer gcc releases support byte-sized atomic accesses on
riscv64 through inline builtins.  In both cases the hard dependency on
libatomic added by GH-11321 isn't useful.

Stop using AC_CHECK_LIB() which is too naive to notice that libatomic
isn't needed.  Instead, PHP_CHECK_FUNC() will retry the check with -latomic
if required.

Closes GH-11790

Author: jcourreges

NEWS

@@ -4,6 +4,8 @@ PHP                                                                        NEWS
 
 - Core:
   . Fixed bug GH-11937 (Constant ASTs containing objects). (ilutov)
+  . Fixed bug GH-11790 (On riscv64 require libatomic if actually needed).
+    (Jeremie Courreges-Anglas)
 
 - DOM:
   . Fix memory leak when setting an invalid DOMDocument encoding. (nielsdos)

configure.ac

@@ -369,11 +369,7 @@ AC_CHECK_LIB(m, sin)
 
 case $host_alias in
   riscv64*)
-    AC_CHECK_LIB(atomic, __atomic_exchange_1, [
-      PHP_ADD_LIBRARY(atomic)
-    ], [
-      AC_MSG_ERROR([Problem with enabling atomic. Please check config.log for details.])
-    ])
+    PHP_CHECK_FUNC(__atomic_exchange_1, atomic)
     ;;
 esac