arch/common-switches: undef _FORTIFY_SOURCE before redefining

[Cyanoxygen]

Otherwise for every invocation the compiler will emit this warning:

<command-line>: warning: "_FORTIFY_SOURCE" redefined
<command-line>: note: this is the location of the previous definition

[Cyanoxygen]

The impact on not undefing is observed on building pcp: pcp needs to test whether functions passed to scandir() should be taking const struct dirent * or struct dirent *. The way it tests this is to see if there are warnings emitted out during compiling the test program.

Without undefing _FORTIFY_SOURCE the compiler emits the warning, thus failing the test, however functions passed to scandir() must take const struct dirent *, resulting a FTBFS.

[Cyanoxygen]

I will report it to the upstream - instead of testing if there's any warning emitted out, it should test if the compiler exited with an error, with -Werror=incompatible-pointer-types.

[MingcongBai]

LGTM

[Cyanoxygen]

Upstream PR: performancecopilot/pcp#2111

[Cyanoxygen]

Upstream PR: performancecopilot/pcp#2111