Need to provide i386 __libc_do_syscall when PROF is defined.
Define OPTIMIZE_FOR_GCC_5 for .S files so that it can be used
in libc-do-syscall.S.
* sysdeps/unix/sysv/linux/i386/libc-do-syscall.S: Replace
__GNUC_PREREQ (5,0) with OPTIMIZE_FOR_GCC_5.
* sysdeps/unix/sysv/linux/i386/sysdep.h (OPTIMIZE_FOR_GCC_5):
Moved before "#ifdef __ASSEMBLER__".