i386: Remove OPTIMIZE_FOR_GCC_5 from Linux libc-do-syscall.S

After commit a78e6a10
("i386: Remove broken CAN_USE_REGISTER_ASM_EBP (bug 28771)"),
it is never defined.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
(cherry picked from commit 6e5c7a1e)