clang: Add support to build eBPF for user specified ARCH

BCC at the moment builds eBPF only considering the local architecture
instead of the one that the user's target kernel is running on.
For cross-compiler environments, the ARCH environment variable is
used to specify which ARCH to build the kernel for. In this patch we
add support to read ARCH and if that's not set, then fallback to
detecting based on local architecture.

This patch borrows some code from a patch doing a similar thing for
eBPF samples in the kenrel that I submitted recently [1]

[1] https://patchwork.kernel.org/patch/9961801/



Signed-off-by: Joel Fernandes <joelaf@google.com>