Makefile: constrain gometalinter memory usage
Makefile: constrain gometalinter memory usage

gometalinter gets OOM-killed on travis:
https://travis-ci.org/google/syzkaller/jobs/462546388

I've run 4 experiments locally and this combination
seems to make memory usage slightly better:

GOGC=50 GOMAXPROCS=1 TIME="%e %M" time gometalinter.v2 ./...
114.35 8274556
122.54 8293580
117.85 8349960

GOGC=50 GOMAXPROCS=1 TIME="%e %M" time gometalinter.v2 ./...
147.45 7512512
150.13 6820408
149.26 7909636

GOGC=100 GOMAXPROCS=2 TIME="%e %M" time gometalinter.v2 ./...
109.73 8332476
114.91 8563776
114.54 8478368

GOGC=50 GOMAXPROCS=2 TIME="%e %M" time gometalinter.v2 ./...
145.43 6726144
147.40 8247952
155.56 7815656

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