[lldb][test] Enable non-address bit WritePointerToMemory test on Linux

First added in llvm#153585 for Darwin only. All Linux AArch64 systems also
have Top Byte Ignore enabled in userspace so the test "just works" there.

FreeBSD has very recently gained Top Byte Ignore support:
freebsd/freebsd-src@4c6c27d

However it's so recent, I don't want to assume it'll be available on
any random FreeBSD system out there.

There isn't really a good place to put this test, so I put it in the
top level of API, next to the other non-address bit test that didn't
have a good home either.

Author: DavidSpickett

lldb/test/API/macosx/arm-pointer-metadata-stripping/Makefile renamed to lldb/test/API/pointer-nonaddressable-bits/Makefile

@@ -5,9 +5,11 @@
 from lldbsuite.test.lldbtest import *
 from lldbsuite.test import lldbutil
 
-
-@skipUnlessDarwin
-@skipIf(archs=no_match(["arm64", "arm64e"]))
+# On AArch64 systems, unused top bits of pointers can be used for other things.
+@skipIf(archs=no_match(["aarch64", "arm64", "arm64e"]))
+# Only run this test on systems where Top Byte Ignore is known to be enabled
+# and widely available (FreeBSD has support but only since recently).
+@skipUnlessPlatform(["linux"] + lldbplatformutil.getDarwinOSTriples())
 class TestArmPointerMetadataStripping(TestBase):
     # Use extra_symbols.json as a template to add a new symbol whose address
     # contains non-zero high order bits set.