Update PR ld/21562 tests for underscore targets

We also need to provide __start_scnfoo and __stop_scnfoo with the extra
leading underscore for underscore targets.

This patch fixed:

FAIL: ld-elf/pr21562k
FAIL: ld-elf/pr21562l
FAIL: ld-elf/pr21562m
FAIL: ld-elf/pr21562n

for metag-linux,

	* testsuite/ld-elf/pr21562c.t: Also provide ___start_scnfoo and
	___stop_scnfoo.
	* testsuite/ld-elf/pr21562d.t: Likewise.

ld/ChangeLog

@@ -1,3 +1,9 @@
+2017-11-24  H.J. Lu  <hongjiu.lu@intel.com>
+
+	* testsuite/ld-elf/pr21562c.t: Also provide ___start_scnfoo and
+	___stop_scnfoo.
+	* testsuite/ld-elf/pr21562d.t: Likewise.
+
 2017-11-23  Martin Storsjo  <martin@martin.st>
 
 	* pe-dll.c (autofilter_liblist): Add libmsvcrt, libmsvcrt-os and

ld/testsuite/ld-elf/pr21562c.t

@@ -1,7 +1,9 @@
 SECTIONS {
     .foo : {
 	PROVIDE (__start_scnfoo = .);
+	PROVIDE (___start_scnfoo = .);
 	*(scnfoo)
 	PROVIDE (__stop_scnfoo = .);
+	PROVIDE (___stop_scnfoo = .);
     }
 }

ld/testsuite/ld-elf/pr21562d.t

@@ -1,5 +1,7 @@
 SECTIONS {
 	PROVIDE (__start_scnfoo = .);
+	PROVIDE (___start_scnfoo = .);
 	.foo : { *(scnfoo) }
 	PROVIDE (__stop_scnfoo = .);
+	PROVIDE (___stop_scnfoo = .);
 }