mesa: re-enable GLX TLS to help i386

mesa's assembly code for i386 only seems to be position independent
when compiled with ELF TLS, so having it disabled causes worse problems
than the edge cases ELF TLS causes.

potentially the assembly code could be fixed (it looks like it not
being safe is a bug), it could also be disabled, but i don't currently
have the means to measure the impact of that.

(nia)