debug/ltrace: Fix HOST_OS and ar

HOST_OS really is the target OS. Allow setting it for configure
via an environment variable.
libltrace.a should have an index:
Allow ar to be set as an environment variable, and generate
an index in this lib.

Reported-by: "Guylhem Aznar" <crossgcc@guylhem.net>
Signed-off-by: "Titus von Boxberg" <titus@v9g.de>

--- gcc-4.3.4/libffi/src/arm/sysv.S	

+++ gcc-4.3.4/libffi/src/arm/sysv.S	

@@ -235,7 +235,7 @@ ARM_FUNC_START ffi_closure_SYSV

 	stmfd	sp!, {ip, lr}

 	UNWIND .save	{r0, lr}

 	add	r2, sp, #8

-	.pad #16

+	UNWIND .pad #16

 	sub	sp, sp, #16

 	str	sp, [sp, #8]

 	add	r1, sp, #8