M1 immediates are limited to 31-bit before they become negative.
* module/mescc/x86_64/as.scm (x86_64:value->r, x86_64:r-cmp-value,
x86_64:r-long-mem-add, x86_64:r-and): Limit M1 immediate to #x80000000.
* lib/tests/scaffold/67-m1-overflow-check.c: New test.
* build-aux/check-mescc.sh (mes_tests): Add it.
