Bazel has a special command line option (--compilation_mode) for specifying how
c artifacts should be compiled. Copy over the implementation without too much
thought on whether they make sense.
See https://docs.bazel.build/versions/main/user-manual.html#flag--compilation_mode.
!!! Due to a regression in zig-cc, debug symbols are available under --compilation_mode opt
```
laurynasl@laurynasl ~/bazel-zig-cc
% bazel build -c opt //test/c:which_libc 2>/dev/null && bazel-bin/test/c/which_libc && gdb -batch --ex 'info line main' bazel-bin/test/c/which_libc
glibc_2.19
Line 4 of "test/c/main.c" starts at address 0x2014f0 <main> and ends at 0x2014f1 <main+1>.
laurynasl@laurynasl ~/bazel-zig-cc
% bazel build -c dbg //test/c:which_libc 2>/dev/null && bazel-bin/test/c/which_libc && gdb -batch --ex 'info line main' bazel-bin/test/c/which_libc
glibc_2.19
Line 4 of "test/c/main.c" starts at address 0x201500 <main> and ends at 0x201504 <main+4>.
laurynasl@laurynasl ~/bazel-zig-cc
% bazel build -c fastbuild //test/c:which_libc 2>/dev/null && bazel-bin/test/c/which_libc && gdb -batch --ex 'info line main' bazel-bin/test/c/which_libc
glibc_2.19
Function "main" not defined.
```
This linker flag causes undefined symbols to be ignored, which is the
default behavior on linux but not macos. This is required when building
shared libraries to use as e.g. Python modules. This also matches what
Bazel's built-in macos cc toolchain does.
Now that the toolchain supports multiple mirrors, we can have more
robust defaults: try ziglang.org first, then dl.jakstys.lt when that one
removes the build we rely on.
This will make builds less prone to inevitable failures of my home
server.
`zig cc` emits `--gc-sections` for the linker, which is incompatbile
with what CGo thinks about linking.
This commit adds a workaround: it will add `--no-gc-sections` to the
linking step if the command is not specified (falling back to the
default behavior of gcc/clang).
Related: https://github.com/golang/go/issues/52690
