🚨 Make things work with GCC 8.3

[HotelCalifornia]

Summary:

arm-none-eabi-8-2019-q3-update was released, which finally bumped arm-none-eabi-gcc to 8.3 officially, so we started running into the missing __sync_synchronize symbol as well. This PR fixes that, as well as a new multiple definitions issue (__malloc_lock/unlock).

The solution to the __sync_synchronize issue is described in #153 (comment).

__malloc_lock and __malloc_unlock were moved from src/system/newlib_stubs.c to their own file src/system/mlock.c, which apparently is how newlib determines overrides.

Motivation:

Since the latest official version of the toolchain now includes 8.3, we can't really just tell people that they should use the official one instead of the preview build arch package managers had access to. We should also be endeavoring to build the kernel against the latest version of the toolchain anyway.

References (optional):

fixes #153, ref #146, ref #134, forum posts: https://www.vexforum.com/t/pros-programming-help/64328 https://www.vexforum.com/t/problem-with-building-files-in-pros-on-a-mac/64326, at least one email

Test Plan:

• compiles with no errors or warnings against arm-none-eabi-8-2019-q3-update
• compiles with no errors or warnings against an older version of the toolchain

[brenapp]

For those looking for temporary solutions until this gets merged into master, you can define your own stub to rid the __sync_synchronize build errors. Create src/stub.c, and add the following code:

void __sync_synchronize(void)
{
    __sync_synchronize();
}

Delete your bin and run pros make again, and the issue should be resolved.

[baylessj]

LGTM 👍 The comment on __sync_synchronize was helpfiul, thanks for writing that.

[Octogonapus]

Can we merge this? Looks like the testing you wanted to do is done, and other people have been using this fix with success.