use explicit dynamic-list rather than symbolic-functions for linking

we have always bound symbols at libc.so link time rather than runtime
to minimize startup-time relocations and overhead of calls through the
PLT, and possibly also to preclude interposition that would not work
correctly anyway if allowed. historically, binding at link-time was
also necessary for the dynamic linker to work, but the dynamic linker
bootstrap overhaul in commit f3ddd17380
made it unnecessary.

our use of -Bsymbolic-functions, rather than -Bsymbolic, was chosen
because the latter is incompatible with public global data; it makes
it incompatible with copy relocations in the main program. however,
not all global data needs to be public. by using --dynamic-list
instead with an explicit list, we can reduce the number of symbolic
relocations left for runtime.

this change will also allow us to permit interposition of specific
functions (e.g. the allocator) if/when we want to, by adding them to
the dynamic list.

configure

@@ -590,10 +590,12 @@ tryldflag LDFLAGS_AUTO -Wl,--no-undefined
 # versions built without shared library support and pcc are broken.
 tryldflag LDFLAGS_AUTO -Wl,--exclude-libs=ALL
 
-# Linking with -Bsymbolic-functions is no longer mandatory for
-# the dynamic linker to work, but enable it if it works as
-# a linking optimization.
-tryldflag LDFLAGS_AUTO -Wl,-Bsymbolic-functions
+# Public data symbols must be interposable to allow for copy
+# relocations, but otherwise we want to bind symbols at libc link
+# time to eliminate startup relocations and PLT overhead. Use
+# --dynamic-list rather than -Bsymbolic-functions for greater
+# control over what symbols are left unbound.
+tryldflag LDFLAGS_AUTO -Wl,--dynamic-list="$srcdir/dynamic.list"
 
 # Find compiler runtime library
 test -z "$LIBCC" && tryldflag LIBCC -lgcc && tryldflag LIBCC -lgcc_eh

dynamic.list

@@ -0,0 +1,35 @@
+{
+environ;
+__environ;
+
+stdin;
+stdout;
+stderr;
+
+timezone;
+daylight;
+tzname;
+__timezone;
+__daylight;
+__tzname;
+
+signgam;
+__signgam;
+
+optarg;
+optind;
+opterr;
+optreset;
+__optreset;
+
+getdate_err;
+
+h_errno;
+
+program_invocation_name;
+program_invocation_short_name;
+__progname;
+__progname_full;
+
+__stack_chk_guard;
+};