fix: standard library selection in toolchain

[jungleraptor]

I noticed that in #92 we inadvertently made it so that the standard library that is selected will always be libstdc++

Here we detect if a builtin sysroot exists to decide whether to choose libstdc++ or libc++:

rules_swiftnav/cc/toolchains/llvm/cc_toolchain_config.bzl

Line 38 in 0775da8

use_libstdcpp = builtin_sysroot != None and not is_darwin

The problem with this is that we pass a select type at the call-site:

rules_swiftnav/cc/toolchains/llvm/x86_64-linux/BUILD.bazel

Line 118 in 0775da8

builtin_sysroot = select({

select statements are not expanded in macros, so builtin_sysroot != None is always true.

Design Notes

To fix I refactored the toolchain config so that the standard library compiler and linker options are set via select statements.

I also made it so that regardless of the config setting - if we're cross compiling, then libstdc++ will be selected.

I also added an extra config setting _use_libstdc++ so that libstdc++ can be used independently of a sysroot.

Testing

• linux x86_64 using libc++ (default)
• linux x86_64 using libstdc++
• linux x86_64 using sysroot (libstdc++)
• linux x86_64 -> linux aarch64 using libstdc++
• darwin x86_64