1.0.41.30: ppc: Add thread-local symbol-value accessor macros.

1.0.41.30: ppc: Add thread-local symbol-value accessor macros.

  * This is basically a straight rip of the x86-64 version.

  * These macros can only be used on static-symbols, so why
aren't we pre-allocating their TLS indices and making them
available at cross-compile time?  If we did, we could elide
a temp register on stores and a memory read on all accesses.