selftests/powerpc: Add test for strlen()

This patch adds a test for strlen()

string.c contains a copy of strlen() from lib/string.c

The test first tests the correctness of strlen() by comparing
the result with libc strlen(). It tests all cases of alignment.

It them tests the duration of an aligned strlen() on a 4 bytes string,
on a 16 bytes string and on a 256 bytes string.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Drop change log from copy of string.c]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/stringloops/Makefile

@@ -6,7 +6,7 @@ EXTRA_SOURCES := ../harness.c
 
 build_32bit = $(shell if ($(CC) $(CFLAGS) -m32 -o /dev/null memcmp.c >/dev/null 2>&1) then echo "1"; fi)
 
-TEST_GEN_PROGS := memcmp_64
+TEST_GEN_PROGS := memcmp_64 strlen
 
 $(OUTPUT)/memcmp_64: memcmp.c
 $(OUTPUT)/memcmp_64: CFLAGS += -m64 -maltivec
@@ -18,6 +18,8 @@ $(OUTPUT)/memcmp_32: CFLAGS += -m32
 TEST_GEN_PROGS += memcmp_32
 endif
 
+$(OUTPUT)/strlen: strlen.c string.c
+
 ASFLAGS = $(CFLAGS)
 
 include ../../lib.mk