selftests/mm: fix powerpc ARCH check

In commit 0518dbe9 ("selftests/mm: fix cross compilation with LLVM")
the logic to detect the machine architecture in the Makefile was changed
to use ARCH, and only fallback to uname -m if ARCH is unset.  However the
tests of ARCH were not updated to account for the fact that ARCH is
"powerpc" for powerpc builds, not "ppc64".

Fix it by changing the checks to look for "powerpc", and change the
uname -m logic to convert "ppc64.*" into "powerpc".

With that fixed the following tests now build for powerpc again:
 * protection_keys
 * va_high_addr_switch
 * virtual_address_range
 * write_to_hugetlbfs

Link: https://lkml.kernel.org/r/20240506115825.66415-1-mpe@ellerman.id.au
Fixes: 0518dbe9 ("selftests/mm: fix cross compilation with LLVM")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mark Brown <broonie@kernel.org>
Cc: <stable@vger.kernel.org>	[6.4+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/Makefile

@@ -12,7 +12,7 @@ uname_M := $(shell uname -m 2>/dev/null || echo not)
 else
 uname_M := $(shell echo $(CROSS_COMPILE) | grep -o '^[a-z0-9]\+')
 endif
-ARCH ?= $(shell echo $(uname_M) | sed -e 's/aarch64.*/arm64/' -e 's/ppc64.*/ppc64/')
+ARCH ?= $(shell echo $(uname_M) | sed -e 's/aarch64.*/arm64/' -e 's/ppc64.*/powerpc/')
 endif
 
 # Without this, failed build products remain, with up-to-date timestamps,
@@ -98,13 +98,13 @@ TEST_GEN_FILES += $(BINARIES_64)
 endif
 else
 
-ifneq (,$(findstring $(ARCH),ppc64))
+ifneq (,$(findstring $(ARCH),powerpc))
 TEST_GEN_FILES += protection_keys
 endif
 
 endif
 
-ifneq (,$(filter $(ARCH),arm64 ia64 mips64 parisc64 ppc64 riscv64 s390x sparc64 x86_64))
+ifneq (,$(filter $(ARCH),arm64 ia64 mips64 parisc64 powerpc riscv64 s390x sparc64 x86_64))
 TEST_GEN_FILES += va_high_addr_switch
 TEST_GEN_FILES += virtual_address_range
 TEST_GEN_FILES += write_to_hugetlbfs