Disable prebuilt_build_tools for non-x86 targets

The prebuilts we have are only for x86_64 .We don't have prebuilts for
other architectures like arm64. Therefore, enable the module only for
the supported arch. This is in preparation for adding arm64 to
linux_bionic OS. Without this change, with HOST_CROSS_OS=linux_bionic
HOST_CROSS_ARCH=arm64, we get

out/soong/make_vars-aosp_arm64.mk:765: error: cannot assign to readonly
variable: SOONG_BISON

because SOONG_BISON is defined twice: once for linux_glibc/x86_64 and
once for linux_bionic/arm64.

Bug: 159685774
Test: m
Change-Id: I2fa5149b05f361e50b773c3ddc461238760df801
diff --git a/Android.bp b/Android.bp
index 910290a..aff7f2a 100644
--- a/Android.bp
+++ b/Android.bp
@@ -12,6 +12,12 @@
     name: "bison",
     deps: ["common/bison/**/*"],
     export_to_make_var: "BISON",
+    enabled: false,
+    arch: {
+        x86_64: {
+            enabled: true,
+        },
+    },
     target: {
         darwin: {
             src: "darwin-x86/bin/bison",
@@ -27,6 +33,12 @@
 prebuilt_build_tool {
     name: "flex",
     export_to_make_var: "LEX",
+    enabled: false,
+    arch: {
+        x86_64: {
+            enabled: true,
+        },
+    },
     target: {
         darwin: {
             src: "darwin-x86/bin/flex",
@@ -40,6 +52,12 @@
 prebuilt_build_tool {
     name: "m4",
     export_to_make_var: "M4",
+    enabled: false,
+    arch: {
+        x86_64: {
+            enabled: true,
+        },
+    },
     target: {
         darwin: {
             src: "darwin-x86/bin/m4",