Increase link concurrency for sanitizer-x86_64-linux-bootstrap bot.

[krasin]

Summary

After sanitizer-x86_64-linux-bootstrap bot upgrade to have more RAM,
it's possible to link more binaries at once. Bumping the limit
from 3 to 20 to speed up the build.

Also, remove unused MAKE_JOBS.

[samsonov]

IIRC we had strong reasons to limit the number of link jobs to just 3: sanitized binaries in debug build (and UBSan bootstrap uses debug) are enormous, and linking them eats many gigs of RAM, hanging the server. eugenis@ did this experiments.

Maybe the server is powerful enough now, but please be cautious.

zorg/buildbot/builders/sanitizers/buildbot_bootstrap.sh

 STAGE2_ASAN_DIR=llvm_build_asan
 STAGE2_MSAN_DIR=llvm_build_msan
 STAGE2_LIBCXX_MSAN_DIR=libcxx_build_msan
 STAGE2_LIBCXX_ASAN_DIR=libcxx_build_asan
 STAGE2_LIBCXX_UBSAN_DIR=libcxx_build_ubsan
 STAGE2_UBSAN_DIR=llvm_build_ubsan
 STAGE3_ASAN_DIR=llvm_build2_asan
 STAGE3_MSAN_DIR=llvm_build2_msan
-MAKE_JOBS=${MAX_MAKE_JOBS:-16}
 LLVM=$ROOT/llvm
-CMAKE_COMMON_OPTIONS="-GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_PARALLEL_LINK_JOBS=3"
+CMAKE_COMMON_OPTIONS="-GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_PARALLEL_LINK_JOBS=20"
 
 if [ "$BUILDBOT_CLOBBER" != "" ]; then
   echo @@@BUILD_STEP clobber@@@
   rm -rf llvm
   rm -rf ${STAGE1_DIR}
 fi
 
 # CMake does not notice that the compiler itself has changed.