Implement _dl_catch_error, _dl_signal_error in libc.so [BZ #16628]

This change moves the main implementation of _dl_catch_error,
_dl_signal_error to libc.so, where TLS variables can be used
directly.  This removes a writable function pointer from the
rtld_global variable.

For use during initial relocation, minimal implementations of these
functions are provided in ld.so.  These are eventually interposed
by the libc.so implementations.  This is implemented by compiling
elf/dl-error-skeleton.c twice, via elf/dl-error.c and
elf/dl-error-minimal.c.

As a side effect of this change, the static version of dl-error.c
no longer includes support for the
_dl_signal_cerror/_dl_receive_error mechanism because it is only
used in ld.so.

sysdeps/unix/sysv/linux/sparc/sparc32/localplt.data

@@ -24,3 +24,6 @@ ld.so: malloc
 ld.so: calloc
 ld.so: realloc
 ld.so: free
+# The TLS-enabled version of these functions is interposed from libc.so.
+ld.so: _dl_signal_error
+ld.so: _dl_catch_error

sysdeps/unix/sysv/linux/sparc/sparc64/localplt.data

@@ -26,3 +26,6 @@ ld.so: malloc
 ld.so: calloc
 ld.so: realloc
 ld.so: free
+# The TLS-enabled version of these functions is interposed from libc.so.
+ld.so: _dl_signal_error
+ld.so: _dl_catch_error