lib: make up for GCC 4.8 builtin expansion bug

This one: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56888
testbench: fix -lm absence when compiled under -Os

It makes Ipc measurably slower, so don't do that. -O2 is the gold standard for
the time being.
ipc: add fast intra-space string transfers

Under core2 KVM this cuts the number of clocks to a tenth.