Rationalize arm-gcc builds in all.sh

[gilles-peskine-arm]

In all.sh, we have arm builds with Arm Compiler, IAR, and GCC. The GCC builds evolved in an ad hoc way and could use some clean up.

• Originally there was just one build with arm-gcc-none-eabi, just to have a non-intel target.
• Then (bn_mul.h: require at least ARMv6 to enable the ARM DSP code #2777) we added a build that's similar to Debian armel (following bn_mul.h: require at least ARMv6 to enable the ARM DSP code #2169 (comment) and subsequent discussion), but not identical because we didn't have the right compiler on our CI.
• Then (Rationalize Travis builds #3218) we added a Cortex-M0+ build mainly to have a convenient, simple measure of size.

This could use some rationalization.

• We should ensure that we're building all of the assembly variants. (Note that some assembly is disabled at -O0.)
• There should be at least one thumb no-assembly build and one non-thumb no-assembly build.
• The builds with MBEDTLS_NO_64BIT_MULTIPLICATION and with MBEDTLS_NO_UDBL_DIVISION should not enable assembly, otherwise we may be missing an occurrence of 64-bit mul/div.
• Review the optimization levels. We definitely want some -Os builds, and we need -O0 on no-assembly builds. Maybe we should have some -O2 builds as well.
• We should use more varied compilers. The “Debian armel” build should use the same compiler (arm-linux-gnueabi-gcc).
• We should add at least one Aarch64 build. Writing the corresponding assembly is out of scope here.
• We should test all of these builds, but that's out of scope here.

The goal of this PR is to rationalize the set of 32-bit arm builds with free compilers in all.sh. If installing the right compilers on our CI systems is not easy, this can be deferred: file a follow-up issue and add a support_build_xxx function in all.sh.