build: remove --unsafe-optimizations flag

The previous commit removes our patch that builds V8 at -O2 rather
than -O3 so there is not much point in keeping the configure switch
around.

The reason it did so was to work around an assortment of compiler and
linker bugs. In particular, certain combinations of g++ and binutils
generate bad or no code when -ffunction-sections or -finline-functions
is enabled (which -O3 implicitly does.)

It was quite the problem back in the day because everyone and his dog
built from source. Now that we have prebuilt binaries and packages
available, there is no longer a pressing need to be so accommodating.

If you experience spurious (or possibly not so spurious) segmentation
faults after this commit, you need to upgrade your compiler/linker
toolchain.