Add bpfbe and bpfle cpu constraints

[amari]

This patch creates two cpu constraints, enabling downstream toolchains the ability to generate eBPF bytecode.

Please note, that compiler target triples are named using bpf* instead of ebpf*.

Compiler	Target Triple	Notes
clang	bpf bpfbe bpfle	Default endianess matches compiler host/"execute" target.
gcc	bpf-unknown-none	-mlittle-endian/-mbig-endian compiler flags. Little endian is the default.
rustc	bpfbe-unknown-none bpfle-unknown-none