on supported platforms build with system clang by default

[spoonincode]

Change Description

compared to clang, building with gcc has shown significant performance degradation particularly for wabt execution. See #7402. Ubuntu 18 & amz2 will now use a system provided clang to build. Compilers used for various platforms:

Platform	pre 1.8	1.8 rc2	this PR
Ubuntu 16.04	clang 4	clang 8 (via pin)
Ubuntu 18.04	clang 4	gcc 7	clang 6
Amazon Linux 2	gcc 7	gcc 7	clang 7
Centos 7	gcc 7	gcc 7	gcc 8
macOS	clang as provided by Xcode

Centos7 will not be upgraded to clang at this time due to the following reasons:

• While the clang 5 provided by centos' SCL compiles eosio today, it won't compile various c++17 tests. Therefore it isn't considered safe to invest in.
• clang 6 is not available in centos' SCL. It is available in Springdale Linux repos (another RHEL clone) but this wasn't considered reasonable to migrate to at the moment
• clang 7 has been released by RH but no one provides built RPMs yet. (in theory it would be great for us to shepherd this in to centos' SCL)
• centos7 never used clang for eosio, so it's not technically a performance regression to keep using it in to 1.8 and later
• EOSVM doesn't suffer from gcc vs clang nearly as badly, so the performance deviation in the future won't be as bad
• Users can still either
  • install llvm-toolset-6.0 or llvm-toolset-7.0 themselves and use it via setting CC and CXX for the build script
  • Use pinned build getting you a clang 8

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions