CMake: Only build turbo-cov with clang + llvm 10
CMake: Only build turbo-cov with clang + llvm 10

turbo-cov can only process clang coverage information, so there's no point building it for other toolchains.

turbo-cov depends on the llvm-10 libs and headers, and does not build against llvm-7.

Bug: b/152192800
Change-Id: I522ecb3adfbd448aa47673c452a83083134a36ca
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44728
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Tested-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>

bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader