fix incorrect comparison loop condition in memmem

the logic for this loop was copied from null-terminated-string logic
in strstr without properly adapting it to work with explicit lengths.

presumably this error could result in false negatives (wrongly
comparing past the end of the needle/haystack), false positives
(stopping comparison early when the needle contains null bytes), and
crashes (from runaway reads past the end of mapped memory).
add arch-generic support for tlsdesc relocations to dynamic linker

this code is non-functional without further changes to link up the
arch-specific reloc types for tlsdesc and add asm implementations of
__tlsdesc_static and __tlsdesc_dynamic.
change dynamic TLS installation strategy to optimize access

previously, accesses to dynamic TLS had to check two conditions before
being able to use a dtv slot: (1) that the module index was within the
bounds of the current dtv size, and (2) that the dynamic tls for the
requested module index was already installed in the dtv.

this commit changes the installation strategy so that, whenever an
attempt is made to access dynamic TLS that's not yet installed in the
dtv, the dynamic TLS for all lower-index modules is also installed.
thus it provides a new invariant: if a given module index is within
the bounds of the current dtv size, we automatically know that its TLS
is installed and directly available. the requirement that the second
condition (above) be checked is eliminated.
optimize i386 ___tls_get_addr asm
add tlsdesc support for i386
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).
add tlsdesc support for x86_64