sh dlsym: fix passing of return address for RTLD_NEXT use

this code dates back to the original commit of the sh port, with no
real clue as to how the bug was introduced. it looks like it was
written to assume the return address was pushed to the stack like on
x86, rather than arriving in the pr special register.

src/ldso/sh/dlsym.s

@@ -5,7 +5,7 @@
 dlsym:
 	mov.l L1, r0
 1:	braf  r0
-	 mov.l @r15, r6
+	 sts pr, r6
 
 .align 2
 L1:	.long __dlsym@PLT-(1b+4-.)