Add HOST_ARCH detection for armv5tel and armv6l

Add HOST_ARCH detection for armv5tel and armv6l

Since 7506c15, HOST_ARCH is now used by the EFI loader even when
CONFIG_SANDBOX is disabled. When cross-compiling, the Makefile defines
HOST_ARCH based on the cross-compiler prefix, but this definition fails
to cover some common compiler prefixes. When cross-compiling U-Boot in
nixpkgs, we use CROSS_COMPILE=armv6l-unknown-linux-gnueabihf-, which
results in HOST_ARCH being undefined and causes a build failure.

Fix this by adding armv6l to the match for ARM. Also add armv5tel,
as this is another possible ARM compiler prefix.

Signed-off-by: ZHANG Yuntian <yt@radxa.com>
Signed-off-by: Ben Wolsieffer <benwolsieffer@gmail.com>