[compiler-rt] Reduce the number of threads in gcov test to avoid failure

Summary:
Patch in D78477 introduced a new test for gcov and this test is failing on arm:
 - http://lab.llvm.org:8011/builders/clang-cmake-thumbv7-full-sh/builds/4752/steps/ninja%20check%202/logs/stdio
  - http://lab.llvm.org:8011/builders/clang-cmake-armv7-full/builds/10501/steps/ninja%20check%202/logs/stdio
So try to fix it in reducing the number of threads.

Reviewers: marco-c

Reviewed By: marco-c

Subscribers: dberris, kristof.beyls, #sanitizers, serge-sans-paille, sylvestre.ledru

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D79621