Change in coreboot[master]: crossgcc/Makefile: don't allow to call buildgcc in parallel

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Change subject: crossgcc/Makefile: don't allow to call buildgcc in parallel
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Patch Set 1: Verified+1

Build Successful

https://qa.coreboot.org/job/coreboot-checkpatch/14698/ : SUCCESS

https://qa.coreboot.org/job/coreboot-toolchain/360/ : SUCCESS

https://qa.coreboot.org/job/coreboot-gerrit/59389/ : SUCCESS