Simplify platform macro definitions in PAL

[am11]

• Delete _M_{ARCH} definitions and replace usage with HOST_{ARCH} and TARGET_{ARCH}.
• Remove redundant ${arch} from filenames in architecture specific directories.
• Remove Arm from PAL_ArmInterlockedOperationBarrier and ArmInterlockedOperationBarrier as they are not ARM-specific anymore.
• Add a few empty stubs for riscv64 so we can run src/coreclr/build-runtime.sh -cross -a riscv64 -component paltests (without running into missing files errors).

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[am11]

@janvorli, the CI issue was fixed after the rebase. PTAL.

[janvorli]

@am11 I am sorry for the delay, I was OOF for the last 7 days.

[janvorli]

LGTM, thank you!

[am11]

@EgorBo, any idea why superpmi leg would fail the osx-arm64 checks when I updated PAL_CS_NATIVE_DATA_SIZE only for linux-arm64 to 104 (osx-arm64 is 120 which hasn't changed):

[16:48:11] Invoking: C:\h\w\9D87090C\p\artifacts\spmi\pintools\1.0\windows\pin.exe -follow_execv -t C:\h\w\9D87090C\p\artifacts\spmi\pintools\1.0\windows\clrjit_inscount_x64\clrjit_inscount.dll -quiet -- C:\h\w\9D87090C\p\superpmi.exe -applyDiff -baseMetricsSummary C:\h\w\9D87090C\t\tmpsfrtx1z4\libraries_tests.pmi.OSX.arm64.checked.mch_base_metrics.csv -diffMetricsSummary C:\h\w\9D87090C\t\tmpsfrtx1z4\libraries_tests.pmi.OSX.arm64.checked.mch_diff_metrics.csv -target arm64 -p -failureLimit 100 C:\h\w\9D87090C\p\base\release\clrjit_universal_arm64_x64.dll C:\h\w\9D87090C\p\diff\release\clrjit_universal_arm64_x64.dll C:\h\w\9D87090C\p\artifacts\spmi\mch\1b9551b8-21f4-4233-9c90-f3eabd6a322b.OSX.arm64\libraries_tests.pmi.OSX.arm64.checked.mch
[16:48:15] ERROR: Unexpected exception c0000005 was thrown.
[16:48:15] ERROR: Unexpected exception c0000005 was thrown.
[16:48:15] ERROR: Method 1 of size 88 failed to load and compile correctly by JIT2 (C:\h\w\9D87090C\p\diff\release\clrjit_universal_arm64_x64.dll).
[16:48:15] ERROR: Method 1 of size 88 failed to load and compile correctly by JIT1 (C:\h\w\9D87090C\p\base\release\clrjit_universal_arm64_x64.dll).

The CI was green before PAL_CS_NATIVE_DATA_SIZE/DAC_CS_NATIVE_DATA_SIZE 116->104 change for linux-arm64.

[EgorBo]

@am11 we had some issues with it and fixed them yesterday, can you rebase your PR?

[janvorli]

@am11 let me retry the superpmi tests one more time and then I'll merge it.