Add missing bsd link libraries (#859)

This mirrors this behavior from the clang driver:

https://github.com/llvm/llvm-project/blob/3e892fad65730692057da27bebb2e34d3ad7bde7/clang/lib/Driver/ToolChains/OpenBSD.cpp#L365-L366

The same does not apply for freebsd (I didn't check netbsd which isn't
supported here right now).

Alternatively we could use clang++ for linking directly instead of
clang, which would add these automatically, but that would make it
harder to disable these in the case you are linking pure C

Fixes https://github.com/bazelbuild/rules_cc/issues/857

Closes #859

COPYBARA_INTEGRATE_REVIEW=https://github.com/bazelbuild/rules_cc/pull/859 from keith:ks/add-missing-openbsd-link-flags 70575fbff89f98fb4eb76db417e035133e8d767f
PiperOrigin-RevId: 977503273
Change-Id: Ifcb127ace43f5bad5f075de8dc7ae21d3182858d

cc/private/toolchain/bsd_cc_toolchain_config.bzl

diff --git a/cc/private/toolchain/bsd_cc_toolchain_config.bzl b/cc/private/toolchain/bsd_cc_toolchain_config.bzl
index d10557a..74891d6 100644
--- a/cc/private/toolchain/bsd_cc_toolchain_config.bzl
+++ b/cc/private/toolchain/bsd_cc_toolchain_config.bzl
@@ -60,6 +60,10 @@
     if cpu not in ("freebsd", "openbsd"):
         fail("unsupported BSD CPU: %s" % cpu)
 
+    extra_default_link_flags = []
+    if cpu == "openbsd":
+        extra_default_link_flags = ["-lc++abi", "-lpthread"]
+
     default_link_flags_feature = feature(
         name = "default_link_flags",
         enabled = True,
@@ -70,9 +74,10 @@
                     flag_group(
                         flags = [
                             "-lc++",
+                            "-lm",
                             "-Wl,-z,relro,-z,now,-z,origin",
                             "-no-canonical-prefixes",
-                        ],
+                        ] + extra_default_link_flags,
                     ),
                 ],
             ),