libc: clear frame pointer on user-context entry

If built with frame-pointer support the backtrace now stops at the user
entry code. Before, the libc kernel stack was trced back too.

repos/libports/src/lib/libc/include/spec/arm/internal/call_func.h

@@ -25,6 +25,7 @@
 {
 	asm volatile ("mov r0, %2;"  /* set arg */
 	              "mov sp, %0;"  /* set stack */
+	              "mov fp, #0;"  /* clear frame pointer */
 	              "mov pc, %1;"  /* call func */
 	              ""
 	              : : "r"(sp), "r"(func), "r"(arg) : "r0");

repos/libports/src/lib/libc/include/spec/x86_32/internal/call_func.h

@@ -26,6 +26,7 @@
 	asm volatile ("movl %2, 0(%0);"
 	              "movl %1, -0x4(%0);"
 	              "movl %0, %%esp;"
+	              "xorl %%ebp, %%ebp;"  /* clear frame pointer */
 	              "call *-4(%%esp);"
 	              : : "r" (sp), "r" (func), "r" (arg));
 	__builtin_unreachable();