Add a new --cross-prefix option to configure for pwsh users.

[AraHaan]

Added the --cross-prefix option into the configure script with comments explaining why.

@madler I am sorry for this PR but I needed this option to properly build a static lib for linking into my shared library for x86, x64, arm, and arm64 builds of gcc/g++ using cross-compiler packages.

Sadly since after forever when one needs to install the cross compilers it results in:

• uninstall of gcc-multilib, normal gcc, and g++
• install of the cross compiler (since they conflict with gcc-multilib so marks them for removal which breaks normal $CC.
  • This means that once they install a single cross-compiler package they must replace all of the C/C++ compilers with cross-compilers for x86, x64, arm, arm64, longarch64, etc.

The fix:

• Adding a --cross-prefix option to configure to resolve this issue.

[mtl1979]

This doesn't work as related variables are already set when command-line options are parsed. This means the variables need to be set again to reflect the changed target architecture.

[AraHaan]

Which is fine as normally I run ./configure again per cross compiler.

[mtl1979]

Which is fine as normally I run ./configure again per cross compiler.

I mean variables AR, RANLIB and NM are still set to host versions of the tools instead of target versions... Although that might make no difference for some targets when the host tools can handle multiple architectures, it is still possible failure point.