fix microblaze asm relocations for shared libc

only @PLT relocations are considered functions for purposes of
-Bsymbolic-functions, so always use @PLT. it should not hurt in the
static-linked case.

src/signal/microblaze/sigsetjmp.s

@@ -2,8 +2,8 @@
 .type sigsetjmp,@function
 sigsetjmp:
 	lwi     r11, r5, 72
-	beqi    r11, setjmp
+	beqi    r11, setjmp@PLT
 	addi    r7, r5, 76
 	add     r6, r0, r0
-	brid    sigprocmask
+	brid    sigprocmask@PLT
 	ori     r5, r0, 2