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