x86/um: Remove -fno-unit-at-a-time workaround for pre-4.0 GCC

Commit

  cafa0010 ("Raise the minimum required gcc version to 4.6")

bumped the minimum GCC version to 4.6 for all architectures.

'$(call cc-option,-fno-unit-at-a-time)' is now dead code since
'$(cc-version) -lt 0400' is always false. Remove it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Richard Weinberger <richard@nod.at>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/1541990120-9643-2-git-send-email-yamada.masahiro@socionext.com