llvm: Do not build libc++ for Linuxbrew
llvm: Do not build libc++ for Linuxbrew

Remove conflicting libraries, libgomp.so and libunwind.so.
Reduce parallelization to avoid build failures.
Fix test for Linuxbrew.

Closes Linuxbrew/homebrew-core#507.

Signed-off-by: Shaun Jackman <b580dab3251a9622aba3803114310c23fdb42900@gmail.com>

Linuxbrew/homebrew-core,maxim-belkin/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core,LinuxbrewTestBot/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,rwhogg/homebrew-core