Generic 32-bit armv8 built as generic armv7 fix

When the following lines are specified in the BoardConfig.mk

TARGET_2ND_ARCH_VARIANT := armv8-a
TARGET_2ND_CPU_VARIANT := generic

The resulted binaries won't be built using 32-bit armv8-a integer
division instructions. Instead, division routines are used because
the build system will set a compiler option "-march=armv7-a", and
hence overwiting the previous "-march=armv8-a", when
TARGET_2ND_CPU_VARIANT is set to generic (or unset).

Bug: 67446726
Test: lunch aosp_arm64; make -j; emulator # boot to home screen
      Also verify -march=armv7-a is not specified in the compiler
      command line and the resulted binary actually uses integer
      division instructions.

Change-Id: I430687aa5a658d2ec9f325a66d849b4c8898c7d5

cc/config/arm_device.go

@@ -77,6 +77,7 @@ var (
 			"-mfpu=vfpv3-d16",
 		},
 		"armv7-a-neon": []string{
+			"-march=armv7-a",
 			"-mfloat-abi=softfp",
 			"-mfpu=neon",
 		},
@@ -88,9 +89,6 @@ var (
 	}
 
 	armCpuVariantCflags = map[string][]string{
-		"": []string{
-			"-march=armv7-a",
-		},
 		"cortex-a7": []string{
 			"-mcpu=cortex-a7",
 			"-mfpu=neon-vfpv4",