docs: use 'c++' to build on FreeBSD
docs: use 'c++' to build on FreeBSD

Building the executor via `gcc executor/executor_freebsd.cc ...`
requires that a GCC package first be installed on the FreeBSD VM image.
The FreeBSD base system comes with Clang already installed, so we can
build via `c++ executor/executor_freebsd.cc ...` and avoid having to
install additional packages.

xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller