builder: protect against unsupported concurrency in lld

[aykevl]

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LLD does not support concurrency, and cannot be run at the same time as compiles. Therefore, use a RWMutex to read-lock compiles and write-lock links.

This commit moves the locking from the full program tests to the builder package itself, to provide a cleaner API boundary: users of builder.Build do not have to think about concurrency. This move also makes the above mentioned compile concurrency possible.

On my system, this results in about 36% faster tests.

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Note: I've been busy with allowing packages to be compiled independently. When that is ready, lldLock will probably be locked per package.

[aykevl]

Hopefully this will fix the Windows issue.

[deadprogram]

It appears to have fixed Windows. However it broke macOS.

[aykevl]

It was a "too many open files" problem, which seems spurious to me. Fixed it by retrying the build.

However, I did find a file that wasn't closed here: https://github.com/tinygo-org/tinygo/pull/799 (that should reduce the chances of this happening).

[aykevl]

I did another update. I don't see why LLD and Clang would interfere with each other or with regular LLVM building/linking so I switched to more fine grained locks instead (for just Clang and LLD, and Clang only on Windows).

[deadprogram]

Seems to still be broken on macOS.

[aykevl]

Ugh, builder/ar.go is really bad at properly closing files when there is an error. That may have triggered it.

[aykevl]

We invoke lld in a separate process now so this is no longer an issue.