Kernel header scrubber: remove unused map entries.

I was going to add riscv64 and define __riscv_xlen to 64 here, but our
"preprocessor" in python can't simpify `MACRO_NAME == 123` expressions
anyway. Rather than add another empty map, remove the other unused maps
and explain why this functionality is still useful as long as the legacy
32-bit ABIs are still with us.

Test: treehugger
Change-Id: I553059942920e7d323a0ca8a6fd5f39bc2d35063

libc/kernel/tools/defaults.py

@@ -55,12 +55,11 @@ kernel_structs_to_remove = {
 # but just generates cleaner results
 kernel_remove_config_macros = True
 
-# maps an architecture to a set of default macros that would be provided by
-# toolchain preprocessor
+# Maps an architecture to a set of default macros that would be provided by
+# the toolchain's preprocessor. Currently only used to remove confusing
+# big-endian junk from the 32-bit arm headers.
 kernel_default_arch_macros = {
     "arm": {"__ARMEB__": kCppUndefinedMacro, "__ARM_EABI__": "1"},
-    "arm64": {},
-    "x86": {},
     }
 
 # Replace tokens in the output according to this mapping.