xcompile: Add a way to specify -march=i586

Instead of instructing users to edit xcompile when they want to build
a quark platform, give the build a way to set -march=586 so that
the quark code will build correctly.  The Quark processor does not
support the instructions introduced with the Pentium 6 architecture.

Change-Id: I0ed69aadc515f86f76800180e0e33bcd75feac5a
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/13552
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins)
Reviewed-by: Leroy P Leahy <leroy.p.leahy@intel.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: FEI WANG <wangfei.jimei@gmail.com>

src/arch/x86/Kconfig

@@ -58,6 +58,12 @@ config ARCH_RAMSTAGE_X86_64
 	bool
 	default n
 
+config USE_MARCH_586
+	def_bool n
+	help
+	  Allow a platform or processor to select to be compiled using
+	  the '-march=i586' option instead of the typical '-march=i686'
+
 # This is an SMP option. It relates to starting up APs.
 # It is usually set in mainboard/*/Kconfig.
 # TODO: Improve description.