HP-UX: Always add /usr/lib to rpath (#10571)

Set CMAKE_PLATFORM_REQUIRED_RUNTIME_PATH in the HP-UX platform file to
tell CMake to pass -Wl,+b,/usr/lib no matter whether RPATH is enabled or
not.  This corrects the behavior of -Wl,+nodefaultrpath to look in this
default library path as documented.

Modules/Platform/HP-UX.cmake

@@ -1,3 +1,5 @@
+SET(CMAKE_PLATFORM_REQUIRED_RUNTIME_PATH /usr/lib)
+
 SET(CMAKE_SHARED_LIBRARY_SUFFIX ".sl")          # .so
 SET(CMAKE_DL_LIBS "dld")
 SET(CMAKE_FIND_LIBRARY_SUFFIXES ".sl" ".so" ".a")