Kokoro: Update from GCC 4.8 to GCC 7.
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 <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Tested-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>

bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader