[llvm-objcopy][Support] move writeToOutput helper function to Support.

writeToOutput function is useful when it is necessary to create different kinds
of streams(based on stream name) and when we need to use a temporary file
while writing(which would be renamed into the resulting file in a success case).
This patch moves the writeToStream helper into the Support library.

Differential Revision: https://reviews.llvm.org/D98426