buffer: optimize buffer.concat performance #3740

Job	Run time
Build slim tarball	2m 16s
aarch64-darwin: with shared libraries	21m 58s
x86_64-darwin: with shared libraries	28m 13s
x86_64-linux: with shared libraries	25m 39s
aarch64-linux: with shared libraries	20m 43s
	1h 38m 49s

Provide feedback