buffer: optimize buffer.concat performance #3136

Job	Run time
Build slim tarball	1m 39s
x86_64-linux: with shared libraries	3m 16s
aarch64-linux: with shared libraries	3m 11s
x86_64-darwin: with shared libraries	3m 18s
aarch64-darwin: with shared libraries	3m 19s
	14m 43s

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