Kokoro: Update from GCC 4.8 to GCC 7.

GCC 4.8 doesn't even understand --std=c++14

Bug: b/147359661
Change-Id: I5189d24638007ffe40e8903a382bccab9ef1c0fc
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39950
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>

tests/kokoro/gcp_ubuntu/continuous.sh

diff --git a/tests/kokoro/gcp_ubuntu/continuous.sh b/tests/kokoro/gcp_ubuntu/continuous.sh
index 486a306..0d879ec 100755
--- a/tests/kokoro/gcp_ubuntu/continuous.sh
+++ b/tests/kokoro/gcp_ubuntu/continuous.sh
@@ -5,6 +5,12 @@
 set -e # Fail on any error.
 set -x # Display commands being run.
 
+# Specify we want to build with GCC 7
+sudo apt-get update
+sudo apt-get install -y gcc-7 g++-7
+sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 100 --slave /usr/bin/g++ g++ /usr/bin/g++-7
+sudo update-alternatives --set gcc "/usr/bin/gcc-7"
+
 # Download all submodules
 git submodule update --init