cpu/Makefile.include.cortexm_common: don't use cortex-m0plus for clang if unsupported

[cgundogan]

My clang version (3.6.2) does not support the cortex-m0plus mcpu type. I have no idea in which version they ~~will/~~have include/ed this, but building any application for the samr21-xpro with llvm/clang breaks for me:

~/r/R/e/gnrc_networking >>> TOOLCHAIN=clang BOARD=samr21-xpro make -B clean all
Building application "gnrc_networking" for "samr21-xpro" with MCU "samd21".

error: unknown target CPU 'cortex-m0plus'

NOTE: I am currently building llvm/clang 3.8 from source and it appears that the m0plus is supported there. So I will just PR this and look what your opinion is on this matter. Close if necessary.

[jnohlgard]

A suggestion: Instead of checking the reported version number, could we check if the -mcpu=cortexm0plus option is supported by checking the exit code from a test call to clang?

[jnohlgard]

I did some messing around on the command line, the command

clang -target arm-none-eabi -mcpu=cortex-m0plus -c -x c /dev/null -o /dev/null 

will return 0 if all options are valid and 1 when there is an invalid -mcpu option (try it with -mcpu=cortex-m8 and check the exit code)

[cgundogan]

Ah okay, I came up with another solution. Which one do you like more?

[jnohlgard]

missing $()

[cgundogan]

addressed

[cgundogan]

addressed @gebart's comment

[jnohlgard]

also redirect stdout and stderr from clang (>/dev/null 2>&1)

• addressed

[cgundogan]

addressed

[jnohlgard]

​ACK, please squash.
Can this go in the release?

[cgundogan]

squashed.

Can this go in the release?

Frankly speaking, I don't know.
How critical is this for others? Do you think many users will build with llvm/clang? Archlinux' version of llvm/clang in the official repos is 3.6.2. I just looked up that ubuntu's defaul is also 3.6.2 @OlegHahm What's your opinion?

[OlegHahm]

Usually, I would say don't import something "new" into master this late in the release process, but on the other hand this shouldn't break anything for our "default toolchain", so I would be okay with merging.

[jnohlgard]

should be ifeq, I think..

[cgundogan]

addressed @gebart's comment (amended)

[cgundogan]

@gebart does your ACK still hold? (long time since last ACK)

[jnohlgard]

ACK holds

[cgundogan]

travis is happy > GO

[sgso]

@cgundogan Does clang know cortex-m0.small-multiply or has an equivalent?

[cgundogan]

@sgso I checked briefly in their doxygen and searched in the github mirror of clang but couldn't find anything equivalent