MFC r302515:
Implement Linux personality() system call mainly due to READ_IMPLIES_EXEC flag.
In Linux if this flag is set, PROT_READ implies PROT_EXEC for mmap().
Linux/i386 set this flag automatically if the binary requires executable stack.
READ_IMPLIES_EXEC flag will be used in the next Linux mmap() commit.
Approved by: re (gjb)