misc: Revert "arch,cpu,mem,sim: Fold arch/locked_mem.hh..."

This reverts commit a3f85217ab,
https://gem5-review.googlesource.com/c/public/gem5/+/48384

The reason for reverting this commit is it causes the Nightly build to
timeout: https://www.mail-archive.com/gem5-dev@gem5.org/msg40344.html

The exact cause of this failure was a stalling with the O3 processor on
ARM. The simulation reaches the following error and repeats until
timeout:

```
build/ARM/arch/arm/isa.cc:2634: warn: context 0: 2136500000 consecutive store conditional failures
```

The "realview-o3-ARM-x86_64-opt" test can replicate this:

```
./main.py run -j8 --uid
SuiteUID:tests/gem5/fs/linux/arm/test.py:realview-o3-ARM-x86_64-opt
```

Change-Id: I9e9a20753c2a25c143e6a73f58716feb41861cde
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49927
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/SConscript

@@ -59,6 +59,7 @@ env.SwitchingHeaders(
     Split('''
         decoder.hh
         isa.hh
+        locked_mem.hh
         page_size.hh
         pcstate.hh
         vecregs.hh