arch/Config.in.x86: add option for -march=x86-64

We already have support for a wide range of x86-64 CPUs, but we don't
have any option to build for the most generic possible x86-64 CPU, as
made available by the -march=x86-64 GCC option.

This commit makes this option available in Buildroot.

It only really makes sense to use as a 64-bit CPU, and not as a 32-bit
one, so we guard it behind BR2_x86_64.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr: it's an x86-64-only CPU]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

arch/Config.in.x86

@@ -81,6 +81,17 @@ config BR2_x86_prescott
 	select BR2_X86_CPU_HAS_SSE
 	select BR2_X86_CPU_HAS_SSE2
 	select BR2_X86_CPU_HAS_SSE3
+config BR2_x86_x86_64
+	bool "x86-64"
+	depends on BR2_x86_64
+	select BR2_X86_CPU_HAS_MMX
+	select BR2_X86_CPU_HAS_SSE
+	select BR2_X86_CPU_HAS_SSE2
+	help
+	  This option corresponds to -march=x86-64, documented as a
+	  "Generic CPU with 64-bit extensions" by the GCC
+	  documentation. It is a 64-bit CPU with MMX, SSE and SSE2
+	  support.
 config BR2_x86_nocona
 	bool "nocona"
 	select BR2_X86_CPU_HAS_MMX
@@ -281,6 +292,7 @@ config BR2_GCC_TARGET_ARCH
 	default "pentium3"	if BR2_x86_pentium3
 	default "pentium4"	if BR2_x86_pentium4
 	default "prescott"	if BR2_x86_prescott
+	default "x86-64"	if BR2_x86_x86_64
 	default "nocona"	if BR2_x86_nocona
 	default "core2"		if BR2_x86_core2
 	default "corei7"	if BR2_x86_corei7