[1.5] script: do not modify CFLAGS in set_cross_vars

[kolyshkin]

Backport of #5464 to release-1.5. Original description follows.

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This is an alternative to #5456.

When cross-building libseccomp, build_libseccomp resets CFLAGS to an empty value (if it was unset) before each architecture, and set_cross_vars exports it. Autoconf only applies its default CFLAGS (-g -O2) when CFLAGS is unset, so the per-architecture libseccomp builds (which release binaries link against, including for the native architecture) end up unoptimized. See #5456 for performance numbers.

The only reason CFLAGS was touched at all is that set_cross_vars puts -m32 -march=... there for 386, which is why callers had to save and restore it. Pass these flags via CC instead (as autoconf documentation suggests for multilib builds), so set_cross_vars no longer modifies CFLAGS, and remove the now-unneeded save/restore from both build-seccomp.sh and release_build.sh.

This way, a user-provided CFLAGS is passed as is, and if it is unset, configure uses its defaults for all architectures, including 386 (which #5456 does not fix, since set_cross_vars makes CFLAGS non-empty there).

Tested libseccomp 2.6.0 configure (on Debian trixie) with the new set_cross_vars:

• 386: CC='x86_64-linux-gnu-gcc -m32 -march=i686', CFLAGS='-g -O2';
• amd64: CC='x86_64-linux-gnu-gcc', CFLAGS='-g -O2'.

[AkihiroSuda]

Marking as a draft as #5464 is not merged.
Please click "ready for review" when ready.