use hidden visibility for call from dlsym to internal __dlsym

fix microblaze asm relocations for shared libc

only @PLT relocations are considered functions for purposes of
-Bsymbolic-functions, so always use @PLT. it should not hurt in the
static-linked case.

microblaze port

based on initial work by rdp, with heavy modifications. some features
including threads are untested because qemu app-level emulation seems
to be broken and I do not have a proper system image for testing.