Document the minimum GCC / LD supported version

[japaric]

As we found out in #84 our linker script is not supported by GCC 4.8 / binutils 2.24. We should document the minimum version that we support.

I know that binutils 2.31 works.

[mullr]

2.27, the one currently in ubuntu 18.04, appears not to work. (gives the same error as #92)

[mvirkkunen]

Also broken on GCC 6.3.1 which is the current version of gcc-arm-none-eabi in the official Ubuntu repositories. Same error as #92. I switched to LLD for the time being.

[adamgreig]

Using the official ARM toolchain releases, I found the error on 2016q3 (5.4.1) and it worked on 2016q4 (6.2.1) and thereafter. Not sure why the Ubuntu 6.3.1 release doesn't work. Possibly ARM had some patches not yet committed upstream for the 6-series build.

It seems like any GCC 7 or later should work.

I think our recommendation should be to just install the latest official ARM release (or use the default LLD of course).

[therealprof]

@adamgreig Funny you should mention this. I also had various troubles in the past specifically with 2016q3... I would definitely second that we recommend using the latest ARM release, that'll reduce the amount of uncertainty considerably.

[japaric]

No longer needed because this crate no longer depends on arm-none-eabi-gcc. 🎉