Modify CXXFLAGS suggestions in BOINC Wiki

[computezrmle]

The BOINC wiki suggests certain CXXFLAGS to be used for compiling the BOINC client:
https://github.com/BOINC/boinc/wiki/CompileClient#linux

For compiling a portable client on Linux in a VirtualBox VM it suggests this:
CXXFLAGS="-O3 -funroll-loops -fforce-addr -ffast-math"

This should be modified to something like this:
CXXFLAGS="-march=x86-64 -O3 -ffast-math"

Reasons

As of v7.1 VirtualBox supports x86-64-v3 if the host CPU is recent enough.
Using -march=x86-64 will create a portable binary being able to run even on older target CPUs regardless whether they are Intel or AMD.
Without that option a "native" binary will be created that may fail on those CPUs.

-funroll-loops should be removed.
According to the gcc manual it may or may not speed up the binary but will definitely enlarge it (which causes cache misses).

-fforce-addr (like -fforce-mem) is not mentioned any more in the gcc manual since v4.3.6.

[davidpanderson]

Done - thanks