Fix atomic 64 b description (backport #17) (#19)

* Fix atomic 64 b description (#17)

(cherry picked from commit 85efa4a)

# Conflicts:
#	CMakeLists.txt

* Fix conflicts

Signed-off-by: Pablo Garrido <pablogs9@gmail.com>

Co-authored-by: Pablo Garrido <pablogs9@gmail.com>

CMakeLists.txt

@@ -5,7 +5,7 @@ project(rcutils)
 option(RCUTILS_NO_THREAD_SUPPORT "Disable thread support." OFF)
 option(RCUTILS_NO_FILESYSTEM "Disable filesystem usage." OFF)
 option(RCUTILS_AVOID_DYNAMIC_ALLOCATION "Disable dynamic allocations." OFF)
-option(RCUTILS_NO_64_ATOMIC "Disable support for 64 bits atomic operations." OFF)
+option(RCUTILS_NO_64_ATOMIC "Enable alternative support for 64 bits atomic operations in platforms with no native support." OFF)
 
 # Default to C11
 if(NOT CMAKE_C_STANDARD)