[x86] Changes and Additions to fix broken x86 build

Add missing syscalls to android/libc/arch-x86/syscalls

Remove #undef HAVE_GETLINE from platform.h
Fixes definition conflict warnings for x86 but is also
suitable for arm as  bionic has had a getline implementation for a
while now

Remove hardcoded CONFIG_CROSS_COMPILER_PREFIX from .config-full
and .config-minimal. This is now appended to  .config as part of
the prepare target in Android.mk

It appears no other changes are needed to other files such as
bbconfigopts.h etc as these are autogenerated when make prepare
is run.

Android.mk:
Added missing syscalls source to BUSYBOX_SRC_FILES when
TARGET_ARCH=x86.
Added appending of the CONFIG_CROSS_COMPILER_PREFIX to the .config
file based on the TARGET_ARCH

Change-Id: I4f550757b3209b852cd5a017e88dce50678e5734