Allow the user to set their own mtune/march flags. There is no reason to force specific flags.

diff '--color=auto' -ur a/bazel/toolchains/cc/mongo_linux/mongo_linux_cc_toolchain_config.bzl b/bazel/toolchains/cc/mongo_linux/mongo_linux_cc_toolchain_config.bzl
--- a/bazel/toolchains/cc/mongo_linux/mongo_linux_cc_toolchain_config.bzl	2026-06-15 18:37:04.000000000 -0400
+++ b/bazel/toolchains/cc/mongo_linux/mongo_linux_cc_toolchain_config.bzl	2026-07-20 17:27:34.493483935 -0400
@@ -2007,7 +2007,6 @@
         tsan_feature,
         ubsan_feature,
         ubsan_third_party_feature,
-        mtune_march_feature,
         compress_debug_disable_feature,
         rpath_override_feature,
         warnings_as_errors_link_feature,