disallow big-endian on aarch64

Some recent work involving SIMD instructions on AArch64 made me wonder
whether we support $SUBJECT.  For reference, AArch64 is bi-endian, but
AFAICT all current AAarch64 buildfarm machines are on macOS, Linux, or
FreeBSD, which appear to require little-endian [0] [1] [2].  I know there
are efforts to support Windows on AAarch64, but that requires
little-endian, too [3].  Given the apparent convergence on little-endian,
IMHO we should require it for Postgres, too.  The attached patch adds some
configure-time checks for this.

[0]
https://developer.apple.com/documentation/apple-silicon/porting-your-macos-apps-to-apple-silicon#Address-Architectural-Differences
[1] https://github.com/torvalds/linux/commit/1cf89b6b
[2] https://github.com/freebsd/freebsd-src/commit/ad0a6546
[3] https://learn.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions?view=msvc-170#endianness

-- 
nathan