gh_actions: enable gcc and clang builds, disable parallel ctest for x86

parallel tests have caused weird problems on x86 with
"illegal instructions" during execution, seems
to work fine on arm instances though

.github/workflows/ccpp.yml

@@ -5,13 +5,12 @@ jobs:
     name: Build and test on x86 Ubuntu 18.04
     strategy:
         matrix:
-          cc: [ gcc ]
+          compiler: [gcc, clang]
     runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v1
       - name: Build srsLTE on x86 Ubuntu 18.04
         run: |
-          export CTEST_PARALLEL_LEVEL=$(nproc --all)
           sudo apt update
           sudo apt install -y build-essential cmake libfftw3-dev libmbedtls-dev libpcsclite-dev libboost-program-options-dev libconfig++-dev libsctp-dev colordiff ninja-build valgrind
           mkdir build && cd build && cmake -DRF_FOUND=True -GNinja .. && ninja && ctest -T memcheck
@@ -19,13 +18,12 @@ jobs:
     name: Build and test on x86 Ubuntu 16.04
     strategy:
         matrix:
-          cc: [ gcc ]
+          compiler: [gcc, clang]
     runs-on: ubuntu-16.04
     steps:
       - uses: actions/checkout@v1
       - name: Build srsLTE on x86 Ubuntu 16.04
         run: |
-          export CTEST_PARALLEL_LEVEL=$(nproc --all)
           sudo apt update
           sudo apt install -y build-essential cmake libfftw3-dev libmbedtls-dev libpcsclite-dev libboost-program-options-dev libconfig++-dev libsctp-dev colordiff ninja-build valgrind
           mkdir build && cd build && cmake -DRF_FOUND=True -GNinja .. && ninja && ctest -T memcheck
@@ -33,6 +31,9 @@ jobs:
   aarch64_ubuntu18_build:
     runs-on: ubuntu-18.04
     name: Build on aarch64
+    strategy:
+      matrix:
+        compiler: [gcc, clang]
     steps:
     - uses: actions/checkout@v1
     - name: Build srsLTE on aarch64