build.config.aarch64: s/COMPAT/ARM32

Use LINUX_GCC_CROSS_COMPILE_ARM32_PREBUILTS_BIN instead of
LINUX_GCC_CROSS_COMPILE_COMPAT_PREBUILTS_BIN since that is
backwards-compatible with older kernel/build revisions.

Change-Id: I10b60db33ffdde3d8147b983121367e5ec6566db
Signed-off-by: Elliot Berman <eberman@codeaurora.org>