make sh clone asm fdpic-compatible

clone calls back to a function pointer provided by the caller, which
will actually be a pointer to a function descriptor on fdpic. the
obvious solution is to have a separate version of clone for fdpic, but
I have taken a simpler approach to go around the problem. instead of
calling the pointed-to function from asm, a direct call is made to an
internal C function which then calls the pointed-to function. this
lets the C compiler generate the appropriate calling convention for an
indirect call with no need for ABI-specific assembly.

arch/sh/src/__shcall.c

+__attribute__((__visibility__("hidden")))
+int __shcall(void *arg, int (*func)(void *))
+{
+	return func(arg);
+}

src/thread/sh/clone.s

@@ -33,10 +33,12 @@ __clone:
 	 nop
 
 1:	! we are the child, call fn(arg)
-	jsr    @r1
-	 mov   r2, r4
+	mov.l  1f, r0
+	mov    r1, r5
+	bsrf   r0
+	 mov    r2, r4
 
-	mov   #1, r3   ! __NR_exit
+2:	mov   #1, r3   ! __NR_exit
 	mov   r0, r4
 	trapa #31
 
@@ -45,3 +47,7 @@ __clone:
 	or   r0, r0
 	or   r0, r0
 	or   r0, r0
+
+.align 2
+.hidden __shcall
+1:	.long __shcall@PCREL+(.-2b)