Use locale 'C' for running tests. Closes #17423
collapse setting and exporting RUST_BENCH into one line
auto merge of #17463 : oskchaitanya/rust/master, r=alexcrichton

Setting LC_ALL to C helps keep gdb's output consistent ('print' gives us expected output). This fixes #17423. I do not have access to a windows/mac machines to test this. I've only tested it on an x86_64 linux box.