[flang][runtime] Make sure to link libexecinfo if it exists (#125344)

Fixes building the backtrace support on FreeBSD/NetBSD/OpenBSD/DragonFly and musl
libc with libexecinfo.

(cherry picked from commit cb2598d)

Author: brad0

flang/runtime/CMakeLists.txt

@@ -59,10 +59,15 @@ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
     )
 endif()
 
+set(linked_libraries FortranDecimal)
+
 # function checks
 find_package(Backtrace)
 set(HAVE_BACKTRACE ${Backtrace_FOUND})
 set(BACKTRACE_HEADER ${Backtrace_HEADER})
+if(HAVE_BACKTRACE)
+  list(APPEND linked_libraries ${Backtrace_LIBRARY})
+endif()
 
 include(CheckCXXSymbolExists)
 include(CheckCXXSourceCompiles)
@@ -271,15 +276,15 @@ if (NOT DEFINED MSVC)
   add_flang_library(FortranRuntime
     ${sources}
     LINK_LIBS
-    FortranDecimal
+    ${linked_libraries}
 
     INSTALL_WITH_TOOLCHAIN
   )
 else()
   add_flang_library(FortranRuntime
     ${sources}
     LINK_LIBS
-    FortranDecimal
+    ${linked_libraries}
   )
   set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreaded)
   add_flang_library(FortranRuntime.static ${sources}