Stop using glibc's asm strlen and use our own.

Mostly done because olta want less dependency on glibc.
It should also make porting a tiny bit simpler.

Testresults, mean values on Haiku from libMicro:
 * with glibc: strlen_10: 0.03859S, strlen_1k: 1.67075S.
 * with strlen.cpp: strlen_10: 0.03854S, strlen_1k: 1.66938S.

So at least on my machine it's possible to beat glibc ;)

src/system/libroot/posix/glibc/arch/x86/Jamfile

@@ -112,7 +112,6 @@ MergeObject posix_gnu_arch_$(TARGET_ARCH)_other.o :
 	mplog.c
 	mul_1.S
 	lshift.S rshift.S
-	strlen.S
 	sub_n.S
 	submul_1.S
 ;

src/system/libroot/posix/string/Jamfile

@@ -28,6 +28,7 @@ MergeObject posix_string.o :
 	strerror.c
 	strlcat.c
 	strlcpy.c
+	strlen.cpp
 	strlwr.c
 	strncat.c
 	strncmp.c