libc: Define new symbol visibility macros

This patch defines a few new macros that can be used to control the
visibility of symbols exported by the C library:

- ENTRY_PRIVATE() can be used in assembly sources to indicate
  that an assembler function should have "hidden" visibility, i.e.
  will never be exported by the C library's shared library.

  This is the equivalent of using __LIBC_HIDDEN__ for a C function,
  but ENTRY_PRIVATE() works like ENTRY(), and must be used with
  END() to tag the end of the function.

- __LIBC_ABI_PUBLIC__ can be used to tag a C functions as being
  part of the C library's public ABI. This is important for a
  few functions that must be exposed by the NDK to maintain
  binary compatibility.

  Once a symbol has been tagged with this macro, it shall
  *never* be removed from the library, even if it becomes
  directly unused due to implementation changes
  (e.g. __is_threaded).

- __LIBC_ABI_PRIVATE__ can be used for C functions that should
  always be exported by the C library because they are used by
  other libraries in the platform, but should not be exposed
  by the NDK. It is possible to remove such symbols from the
  implementation if all callers are also modified.

+ Add missing END() assembly macro for x86

Change-Id: Ia96236ea0dbec41d57bea634b39d246b30e5e234

libc/arch-arm/include/machine/asm.h

@@ -97,6 +97,12 @@
 #define	ASENTRY_NP(y)	_ENTRY(_ASM_LABEL(y))
 #define	ASEND(y)	_END(_ASM_LABEL(y))
 
+#ifdef __ELF__
+#define ENTRY_PRIVATE(y)  ENTRY(y); .hidden _C_LABEL(y)
+#else
+#define ENTRY_PRIVATE(y)  ENTRY(y)
+#endif
+
 #define	ASMSTR		.asciz
 
 #if defined(__ELF__) && defined(PIC)

libc/arch-x86/include/machine/asm.h

@@ -103,6 +103,12 @@
 #define _ENTRY(x) \
 	.text; _ALIGN_TEXT; .globl x; .type x,@function; x:
 
+#define _ASM_SIZE(x)    .size x, .-x;
+
+#define _END(x) \
+	.fnend; \
+	_ASM_SIZE(x)
+
 #ifdef GPROF
 # define _PROF_PROLOGUE	\
 	pushl %ebp; movl %esp,%ebp; call PIC_PLT(mcount); popl %ebp
@@ -112,8 +118,12 @@
 
 #define	ENTRY(y)	_ENTRY(_C_LABEL(y)); _PROF_PROLOGUE
 #define	NENTRY(y)	_ENTRY(_C_LABEL(y))
+#define	END(y)		_END(_C_LABEL(y))
 #define	ASENTRY(y)	_ENTRY(_ASM_LABEL(y)); _PROF_PROLOGUE
 
+#define ENTRY_PRIVATE(y)  ENTRY(y); .hidden _C_LABEL(y)
+
+
 #define	ALTENTRY(name)	.globl _C_LABEL(name); _C_LABEL(name):
 
 #define	ASMSTR		.asciz

libc/include/sys/cdefs_elf.h

@@ -96,9 +96,24 @@
 #endif
 
 /* GCC visibility helper macro */
+/* This must be used to tag non-static functions that are private, i.e.
+ * never exposed by the shared library. */
 #define __LIBC_HIDDEN__							\
 	__attribute__ ((visibility ("hidden")))
 
+/* This must be used to tag non-static functions that are public, i.e.
+ * exposed by the shared library, and part of the stable NDK ABI */
+#define __LIBC_ABI_PUBLIC__ \
+        __attribute__ ((visibility ("default")))
+
+/* This must be used to tag non-static functions that must be exported
+ * by the shared library, but whose implementation is private to the
+ * platform. For now this is equivalent to __LIBC_ABI_PUBLIC__, but we
+ * may want to change this later.
+ */
+#define __LIBC_ABI_PRIVATE__ \
+        __attribute__ ((visibility ("default")))
+
 #define	__IDSTRING(_n,_s)		__SECTIONSTRING(.ident,_s)
 
 #define	__RCSID(_s)			__IDSTRING(rcsid,_s)