armv5 build is broken

Summary

Building for architecture ARM v5TE fails in the configure step with message:

checking build system type... armv5tel-unknown-linux-gnueabi
checking host system type... armv5tel-unknown-linux-gnueabi
checking for gcc... gcc
checking whether the C compiler works... yes
[...]
checking for inline... inline
checking for working volatile... yes
checking for flexible array members... yes
checking for yield instruction support... configure: error: no, try adding -march=native or -march=armv7-a to CFLAGS (see PLATFORMS.md for more information)

BIND version used

Commit d134dd9c

Steps to reproduce

On a ARMv5 platform running ArchLinuxARM, simply running:

$ ./configure

is enough.

What is the current bug behavior?

The configure script complains that there is no support for the YIELD instruction, and the build stops there.

What is the expected correct behavior?

BIND9 should not need to check for the YIELD instruction when building for the ARMv5 architecture.

The YIELD instruction (defined since ARMv6K) is an indication to the hardware with no architectural effect (other than a NOP). It may improve performance in a multithread context, if the hardware implements this instruction (e.g. the hardware could increase the memory access priority for the other hardware threads).

In any case, the YIELD instruction is not required for the program to behave correctly. So the absence of support for this instruction should not cause the build to fail.

Possible fixes

0001-Do-not-use-the-YIELD-instruction-for-ARMv5.patch