pie: Mark __export_*() functions as externally_visible

GCC's lto source:
> To avoid this problem the compiler must assume that it sees the
> whole program when doing link-time optimization.  Strictly
> speaking, the whole program is rarely visible even at link-time.
> Standard system libraries are usually linked dynamically or not
> provided with the link-time information.  In GCC, the whole
> program option (@option{-fwhole-program}) asserts that every
> function and variable defined in the current compilation
> unit is static, except for function @code{main} (note: at
> link time, the current unit is the union of all objects compiled
> with LTO).  Since some functions and variables need to
> be referenced externally, for example by another DSO or from an
> assembler file, GCC also provides the function and variable
> attribute @code{externally_visible} which can be used to disable
> the effect of @option{-fwhole-program} on a specific symbol.

As far as I read gcc's source, ipa_comdats() will avoid placing symbols
that are either already in a user-defined section or have
externally_visible attribute into new optimized gcc sections.

Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>

include/common/compiler.h

@@ -30,6 +30,17 @@
 #define __always_unused __attribute__((unused))
 #define __must_check	__attribute__((__warn_unused_result__))
 
+#ifndef __has_attribute
+#define __has_attribute(x) 0
+#endif
+
+/* Not supported by clang */
+#if __has_attribute(__externally_visible__)
+#define __visible __attribute__((__externally_visible__))
+#else
+#define __visible
+#endif
+
 #define __section(S) __attribute__((__section__(#S)))
 
 #ifndef __always_inline