Revert "[mips] revert r366664 - flip mips back from -O2 to -O"

This reverts commit bd72252aace382921840ddbceea712b96f4ad242.

The commit at hand breaks the build for all mips targets and does not
have a one-liner fix.

make[5]: "/usr/src/share/mk/sys.mk" line 169: Malformed conditional (${MACHINE_CPUARCH} == "mips" && ${COMPILER_TYPE} == "gcc")