[Cell/B.E.] Fix wrong relocation for TLS variable offset

[Cell/B.E.] Fix wrong relocation for TLS variable offset

When debugging Cell/B.E. code, the cross-architecture frame unwinding
works by accessing data structures refered to by a thread-local variable
in the inferior.  While accessing this variable via minimal symbol,
code would use BMSYMBOL_VALUE_ADDRESS to determine the offset of the
variable in the thread-local storage block.

This is incorrect, since it adds any relocation offset of the shared
library defining the TLS variable.  That offset would be OK when
accessing the initial copy present in the .tbss section, but it must
not be applied to the offset of the variable in the TLS block at
runtime.  Depending on whether or not the libspe2.so library was
prelinked, access to the variable would fail due to the incorrectly
relocated offset.

ChangeLog:

	* ppc-linux-tdep.c (ppc_linux_spe_context_lookup): Do not
	attempt to relocate a TLS variable offset.