This changes the automatic detection of -fstack-protector, -D_FORTIFY_SOURCE, and -mbranch-protection to write to $hardenflags instead of $XCFLAGS. The definition of $cflags is changed to "$hardenflags $orig_cflags $optflags $debugflags $warnflags" to match. Furthermore, these flags are _prepended_ to $hardenflags, rather than appended. The implications of doing this are as follows: * If a CRuby builder specifies cflags="-mbranch-protection=foobar" at the ./configure script, and the configure script detects that -mbranch-protection=pac-ret is accepted, then GCC will be invoked as "gcc -mbranch-protection=pac-ret -mbranch-protection=foobar". Since the last flags take precedence, that means that user-supplied values of these flags in $cflags will take priority. * Likewise, if a CRuby builder explicitly specifies "hardenflags=-mbranch-protection=foobar", because we _prepend_ to $hardenflags in our autoconf script, we will still invoke GCC as "gcc -mbranch-protection=pac-ret -mbranch-protection=foobar". * If a CRuby builder specifies CFLAGS="..." at the configure line, automatic detection of hardening flags is ignored as before. * C extensions will _also_ be built with hardening flags now as well (this was not the case by default before because the detected flags went into $XCFLAGS). Additionally, as part of this work, I changed how the detection of PAC/BTI in Context.S works. Rather than appending the autodetected option to ASFLAGS, we simply compile a set of test programs with the actual CFLAGS in use to determine what PAC/BTI settings were actually chosen by the builder. Context.S is made aware of these choices through some custom macros. The result of this work is that: * Ruby will continue to choose some sensible defaults for hardening options for the C compiler * Distributors are able to specify CFLAGS that are consistent with their distribution and override these defaults * Context.S will react to whatever -mbranch-protection is actually in use, not what was autodetected * Extensions get built with hardening flags too. [Bug #20154] [Bug #20520]