x86: Switch get_pc_thunk to comdat

Switch the __x86.get_pc_thunk.{bx,cx} functions from .gnu.linkonce to
comdat section groups. lld doesn't implement .gnu.linkonce and will
instead discard the input sections. Currently, it might produce a
faulty binary that has no get_pc_thunk function in it, which would
crash.

Normally, these functions are generated by GCC and are present in many
object files compiled with PIC. Clang doesn't use them, and instead
initializes the PIC base register with a "call 1f; 1: pop %ebx" pair.

Bug: http://b/154376560
Bug: https://bugs.llvm.org/show_bug.cgi?id=45594
Test: bionic-unit-tests
Change-Id: I3e4857aecdc281b32232270bff4880433e5b6afa

libc/Android.bp

@@ -906,6 +906,7 @@ cc_library_static {
                 "arch-x86/bionic/setjmp.S",
                 "arch-x86/bionic/syscall.S",
                 "arch-x86/bionic/vfork.S",
+                "arch-x86/bionic/__x86.get_pc_thunk.S",
 
                 // ssse3 functions
                 "arch-x86/atom/string/ssse3-strcat-atom.S",

libc/arch-x86/atom/string/sse2-memset-atom.S

@@ -77,15 +77,6 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     /* We loaded the jump table and adjusted EDX. Go.  */	\
     jmp		*%ebx
 
-	.section	.gnu.linkonce.t.__x86.get_pc_thunk.bx,"ax",@progbits
-	.globl	__x86.get_pc_thunk.bx
-	.hidden	__x86.get_pc_thunk.bx
-	ALIGN(4)
-	.type	__x86.get_pc_thunk.bx,@function
-__x86.get_pc_thunk.bx:
-	movl	(%esp), %ebx
-	ret
-
 ENTRY(__memset_chk_atom)
   ENTRANCE
 

libc/arch-x86/bionic/__x86.get_pc_thunk.S

@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+// Typically GCC outputs functions like these into any object file that needs a PIC base register,
+// and one of the copies for each register is used. Clang doesn't use these functions, but some of
+// the quasi-hand-written assembly in the optimized string routines does.
+
+    .section .text.__x86.get_pc_thunk.bx,"axG",@progbits,__x86.get_pc_thunk.bx,comdat
+    .globl __x86.get_pc_thunk.bx
+    .hidden __x86.get_pc_thunk.bx
+    .p2align 4
+    .type __x86.get_pc_thunk.bx,@function
+__x86.get_pc_thunk.bx:
+    .cfi_startproc
+    movl (%esp), %ebx
+    ret
+    .cfi_endproc
+
+    .section .text.__x86.get_pc_thunk.cx,"axG",@progbits,__x86.get_pc_thunk.cx,comdat
+    .globl __x86.get_pc_thunk.cx
+    .hidden __x86.get_pc_thunk.cx
+    .p2align 4
+    .type __x86.get_pc_thunk.cx,@function
+__x86.get_pc_thunk.cx:
+    .cfi_startproc
+    movl (%esp), %ecx
+    ret
+    .cfi_endproc

libc/arch-x86/silvermont/string/sse2-memset-slm.S

@@ -77,15 +77,6 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     /* We loaded the jump table and adjusted EDX. Go.  */	\
     jmp		*%ebx
 
-	.section	.gnu.linkonce.t.__x86.get_pc_thunk.bx,"ax",@progbits
-	.globl	__x86.get_pc_thunk.bx
-	.hidden	__x86.get_pc_thunk.bx
-	ALIGN(4)
-	.type	__x86.get_pc_thunk.bx,@function
-__x86.get_pc_thunk.bx:
-	movl	(%esp), %ebx
-	ret
-
 ENTRY(__memset_chk_generic)
   ENTRANCE