cpu/mips32r2: remove nomips16 attribute from _mips_handle_exception

[kaspar030]

Contribution description

A note says "The nomips16 attribute should not really be needed,
it works around a toolchain issue in 2016.05-03."

In fact, in 2018.09-03, the attribute leads to this error:

cpu/mips32r2_common/thread_arch.c:191:1: error: ‘_mips_handle_exception’ redeclared with conflicting ‘nomips16’ attributes

This this commit removes the attribute.

Testing procedure

Successful build and basic test on any mips32r2 board should suffice.

Issues/PRs references

Needed for RIOT-OS/riotdocker#76

[francois-berder]

ACK. I compiled hello-world with GCC toolchain 2017.10-05 and this is necessary to compile any programs for MIPS CPU with toolchains newer than 2016.05-03.

[benpicco]

Tested by @francois-berder
And the build was broken before anyway.

[kaspar030]

There's a logistics problem:

• this PR doesn't build with current riotdocker compiler
• update MIPS toolchain version riotdocker#76 doesn't pass CI because it breaks compiles with the attribute set (travis for riotdocker tests building the latest release)

[kaspar030]

There's a logistics problem:

I propose:

• disable mips temporarily
• bypass CI for update MIPS toolchain version riotdocker#76, merge it
• update CI workers
• re-enable mips

If this is somewhat coordinated, mips would only be disabled for a couple of hours.

[kaspar030]

Or, guard the attribute by "GNUC <= 4", then backport the fix. That should make the current RIOT CI pass. The riotdocker CI might need to be changed to test against the release branch, not the release tag.

[kaspar030]

@francois-berder could you give this another shot?

[kaspar030]

Ok, this is with a newer toolchain:

[kaspar@ng riot/examples/hello-world (mips_remove_nomips16)]$ RIOT_CI_BUILD=1 BOARD=pic32-wifire make clean all -j8
Building application "hello-world" for "pic32-wifire" with MCU "mips_pic32mz".

objcopy: --change-section-lma .gcc_except_table+0xffffffff80000000 never used
   text    data     bss     dec     hex filename
 111440    3017    5056  119513   1d2d9 /home/kaspar/src/riot/examples/hello-world/bin/pic32-wifire/hello-world.elf
[kaspar@ng riot/examples/hello-world (mips_remove_nomips16)]$ mips-mti-elf-gcc --version
mips-mti-elf-gcc (Codescape GNU Tools 2018.09-03 for MIPS MTI Bare Metal) 6.3.0
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[kaspar@ng riot/examples/hello-world (mips_remove_nomips16)]$ 

This is with the current docker image:

[kaspar@ng riot/examples/hello-world (mips_remove_nomips16)]$ BUILD_IN_DOCKER=1 RIOT_CI_BUILD=1 BOARD=pic32-wifire make clean all -j8
Launching build container using image "riot/riotbuild:latest".
docker run --rm -t -u "$(id -u)" \
    -v '/usr/share/zoneinfo/Europe/Berlin:/etc/localtime:ro' -v '/home/kaspar/src/riot:/data/riotbuild/riotbase' -e 'RIOTBASE=/data/riotbuild/riotbase' -e 'CCACHE_BASEDIR=/data/riotbuild/riotbase' -e 'BUILD_DIR=/data/riotbuild/riotbase/build' -e 'RIOTPROJECT=/data/riotbuild/riotbase' -e 'RIOTCPU=/data/riotbuild/riotbase/cpu' -e 'RIOTBOARD=/data/riotbuild/riotbase/boards' -e 'RIOTMAKE=/data/riotbuild/riotbase/makefiles' -v /home/kaspar/.gitcache:/data/riotbuild/gitcache -e GIT_CACHE_DIR=/data/riotbuild/gitcache   \
    -e 'BOARD=pic32-wifire' -e 'RIOT_CI_BUILD=1' \
    -w '/data/riotbuild/riotbase/examples/hello-world/' \
    'riot/riotbuild:latest' make all  
Building application "hello-world" for "pic32-wifire" with MCU "mips_pic32mz".

objcopy: --change-section-lma .gcc_except_table+0xffffffff80000000 never used
   text    data     bss     dec     hex filename
 108828    2788    6024  117640   1cb88 /data/riotbuild/riotbase/examples/hello-world/bin/pic32-wifire/hello-world.elf
[kaspar@ng riot/examples/hello-world (mips_remove_nomips16)]$

So the guard is working. I'll squash, @benpicco could you re-ACK?

[benpicco]

@kaspar030 the ACK is still valid, but so is the whitespace issue brought up by Travis - fix that and you should be good to go.

[kaspar030]

Backport provided in #12288