Regres: support GCOV_PREFIX for collecting coverage
Regres: support GCOV_PREFIX for collecting coverage

If you compile SwiftShader with gcc and the --coverage flag, the build
will contain coverage instrumentation. However, the coverage
instrumentation races when multiple instances of SwiftShader run in
parallel. This change makes Regres replace the string "PROC_ID" inside
the GCOV_PREFIX environment variable with an ID that is unique between
parallel dEQP processes. This allows the user to prevent races using
e.g. GCOV_PREFIX=/tmp/cov/PROC_ID

Change-Id: I227896053c5a0ea34dc8acc282014970a626785d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38928
Reviewed-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>
Tested-by: Paul Thomson <4c6a0f58929afb5de350dd38bc89483c6cc92323@google.com>

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