tools/testing/selftests/vm: fix build error

tools/testing/selftests/vm: fix build error

Only x86 and PowerPC implement the pkey-xxx.h, and an error was reported
when compiling protection_keys.c.

Add a Arch judgment to compile "protection_keys" in the Makefile.

If other arch implement this, add the arch name to the Makefile.
eg:
    ifneq (,$(findstring $(ARCH),powerpc mips ... ))

Following build errors:

    pkey-helpers.h:93:2: error: #error Architecture not supported
     #error Architecture not supported
    pkey-helpers.h:96:20: error: `PKEY_DISABLE_ACCESS' undeclared
     #define PKEY_MASK (PKEY_DISABLE_ACCESS | PKEY_DISABLE_WRITE)
                        ^
    protection_keys.c:218:45: error: `PKEY_DISABLE_WRITE' undeclared
     pkey_assert(flags & (PKEY_DISABLE_ACCESS | PKEY_DISABLE_WRITE));
                                                ^

Signed-off-by: Xingxing Su <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Sandipan Das <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Brian Geffon <[email protected]>
Cc: Mina Almasry <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>