Obsolete the --disable-elf-tls configure option

[alexcrichton]

This option was added to support OSX 10.6, but it's not ideal as it requires that a separate toolchain of Rust is downloaded to compile Rust code for Firefox on OSX. Ideally our support would look something like this:

• The compiler recognizes the "standard method" of specifying that OSX 10.6 is being targeted (not the current version). I believe that this is an environment variable?
• The compiler will then generate an appropriate #[cfg] when this is set to allow the crate to know that it's targeting 10.6 (or whatever the relevant OSX target is)
• The standard library is then compiled against 10.6 always
• The macros in the standard library will then select the "best implementation" based on the context they're built into.

This will incur a perf hit for the standard library itself, but it would allow us to go back to "one distribution works everywhere" which is much nicer than "go build a separate copy of Rust to work on 10.6".

cc @rillian

[rillian]

For Firefox we use MACOSX_DEPLOYMENT_TARGET=10.6 in the environment. This is supported by clang, and overridden by the command line -mmacosx-version-min switch.

I guess that would become #[cfg(macosx_deployment_target)] or maybe #[cfg(target_version)].

MACOSX_DEPLOYMENT_TARGET
           If -mmacosx-version-min is unspecified, the default deployment target
           is read from this environment variable.  This option only affects
           darwin targets.

To match clang's behaviour here rustc would include two versions of the standard library and selectively link to the one with or without TLS based on this variable. This is still a step in that direction, though, so I'm fine with the proposed change. "One distribution" is definitely better for our use in Firefox.