x86: Add --enable-cet=permissive
    
    When CET is enabled, it is an error to dlopen a non CET enabled shared
    library in CET enabled application.  It may be desirable to make CET
    permissive, that is disable CET when dlopening a non CET enabled shared
    library.  With the new --enable-cet=permissive configure option, CET is
    disabled when dlopening a non CET enabled shared library.
    
    Add DEFAULT_DL_X86_CET_CONTROL to config.h.in:
    
     /* The default value of x86 CET control.  */
     #define DEFAULT_DL_X86_CET_CONTROL cet_elf_property
    
    which enables CET features based on ELF property note.
    
    --enable-cet=permissive it to
    
     /* The default value of x86 CET control.  */
     #define DEFAULT_DL_X86_CET_CONTROL cet_permissive
    
    which enables CET features permissively.
    
    Update tst-cet-legacy-5a, tst-cet-legacy-5b, tst-cet-legacy-6a and
    tst-cet-legacy-6b to check --enable-cet and --enable-cet=permissive.
    
    (cherry picked from commit 9e38f455a6c602be86b7b5a8d6523cbdcd7ec051)