Add a 32-bit x86_64 arch variant

The current 32-bit configuration for generic x86_64 targets inherits some
variables (SSE4 support) from the 64-bit configuration, and overrides
the make variables used for other configurations (SSSE3). Ideally, these
would be using different variables, but until then, unify the
configuration for x86_64 targets so that everything is consistent.

Bug: 28694691
Change-Id: I47e67299d4c632e7491d7e73dc0fc6480ef08006

core/combo/arch/x86/x86_64.mk

@@ -0,0 +1,18 @@
+# This file is used as the second (32-bit) architecture when building a generic
+# x86_64 64-bit platform image. (full_x86_64-eng / sdk_x86_64-eng)
+#
+# The generic 'x86' variant cannot be used, since it resets some flags used
+# by the 'x86_64' variant.
+
+ARCH_X86_HAVE_SSSE3 := true
+ARCH_X86_HAVE_MOVBE := false # Only supported on Atom.
+ARCH_X86_HAVE_POPCNT := true
+ARCH_X86_HAVE_SSE4 := true
+ARCH_X86_HAVE_SSE4_1 := true
+ARCH_X86_HAVE_SSE4_2 := true
+
+
+# Some intrinsic functions used by libcxx only exist for prescott or newer CPUs.
+arch_variant_cflags := \
+    -march=prescott \
+

target/board/generic_x86_64/BoardConfig.mk

@@ -13,7 +13,7 @@ TARGET_PRELINK_MODULE := false
 
 TARGET_2ND_CPU_ABI := x86
 TARGET_2ND_ARCH := x86
-TARGET_2ND_ARCH_VARIANT := x86
+TARGET_2ND_ARCH_VARIANT := x86_64
 
 TARGET_USES_64_BIT_BINDER := true