eliminate use of SHARED macro in __tls_get_addr

this was only a tiny optimization, and static-linked binaries should
not be calling __tls_get_addr anyway since the linker is supposed to
perform relaxation, resulting in use of the local-exec TLS model.

fix local-dynamic model TLS on mips and powerpc

the TLS ABI spec for mips, powerpc, and some other (presently
unsupported) RISC archs has the return value of __tls_get_addr offset
by +0x8000 and the result of DTPOFF relocations offset by -0x8000. I
had previously assumed this part of the ABI was actually just an
implementation detail, since the adjustments cancel out. however, when
the local dynamic model is used for accessing TLS that's known to be
in the same DSO, either of the following may happen:

1. the -0x8000 offset may already be applied to the argument structure
passed to __tls_get_addr at ld time, without any opportunity for
runtime relocations.

2. __tls_get_addr may be used with a zero offset argument to obtain a
base address for the module's TLS, to which the caller then applies
immediate offsets for individual objects accessed using the local
dynamic model. since the immediate offsets have the -0x8000 adjustment
applied to them, the base address they use needs to include the
+0x8000 offset.

it would be possible, but more complex, to store the pointers in the
dtv[] array with the +0x8000 offset pre-applied, to avoid the runtime
cost of adding 0x8000 on each call to __tls_get_addr. this change
could be made later if measurements show that it would help.

fix inconsistent visibility for internal __tls_get_new function

at the point of call it was declared hidden, but the definition was
not hidden. for some toolchains this inconsistency produced textrels
without ld-time binding.

separate __tls_get_addr implementation from dynamic linker/init_tls

such separation serves multiple purposes:

- by having the common path for __tls_get_addr alone in its own
function with a tail call to the slow case, code generation is
greatly improved.

- by having __tls_get_addr in it own file, it can be replaced on a
per-arch basis as needed, for optimization or ABI-specific purposes.

- by removing __tls_get_addr from __init_tls.c, a few bytes of code
are shaved off of static binaries (which are unlikely to use this
function unless the linker messed up).