This is very cache-unfriendly, because quite a lot of work is done
during the final linking stage. Let's see how this improves performance
of fastbuild.
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.
```
Jeremy Volkman
Motiejus Jakštys