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_compiler_flags.bzl b/bazel/toolchains/cc/mongo_linux/mongo_compiler_flags.bzl
--- a/bazel/toolchains/cc/mongo_linux/mongo_compiler_flags.bzl	2026-05-13 19:17:03.000000000 -0400
+++ b/bazel/toolchains/cc/mongo_linux/mongo_compiler_flags.bzl	2026-05-29 12:30:08.550864554 -0400
@@ -364,7 +339,6 @@
     COMPRESS_DEBUG_COPTS +
     DEBUG_TYPES_SECTION_FLAGS +
     IMPLICIT_FALLTHROUGH_COPTS +
-    MTUNE_MARCH_COPTS +
     DISABLE_SOURCE_WARNING_AS_ERRORS_COPTS +
     THIN_LTO_FLAGS +
     SYMBOL_ORDER_COPTS +