Avoid increased alignment of TLS segments on Fuchsia

This is a temporary workaround for Fuchsia's libc not supporting
TLS segments with alignments greater than 32 bytes. It should
be reverted ASAP following the fix to libc.

Author: cramertj

src/librustc_codegen_llvm/consts.rs

@@ -345,7 +345,7 @@ pub fn codegen_static<'a, 'tcx>(
         if attrs.flags.contains(CodegenFnAttrFlags::THREAD_LOCAL) {
             llvm::set_thread_local_mode(g, cx.tls_model);
 
-            // Do not allow LLVM to change the alignment of a TLS on macOS.
+            // Do not allow LLVM to change the alignment of a TLS on macOS and Fuchsia.
             //
             // By default a global's alignment can be freely increased.
             // This allows LLVM to generate more performant instructions
@@ -355,6 +355,10 @@ pub fn codegen_static<'a, 'tcx>(
             // respect any alignment given on the TLS (radar 24221680).
             // This will violate the alignment assumption, and causing segfault at runtime.
             //
+            // Fuchsia's libc currently does not support greater than 16-byte alignment
+            // of TLS segments, so this hack is also enabled temporarily for Fuchsia targets
+            // until libc is fixed.
+            //
             // This bug is very easy to trigger. In `println!` and `panic!`,
             // the `LOCAL_STDOUT`/`LOCAL_STDERR` handles are stored in a TLS,
             // which the values would be `mem::replace`d on initialization.
@@ -374,7 +378,9 @@ pub fn codegen_static<'a, 'tcx>(
             // will use load-unaligned instructions instead, and thus avoiding the crash.
             //
             // We could remove this hack whenever we decide to drop macOS 10.10 support.
-            if cx.tcx.sess.target.target.options.is_like_osx {
+            if cx.tcx.sess.target.target.options.is_like_osx ||
+                (cx.tcx.sess.target.target.target_os == "fuchsia")
+            {
                 let sect_name = if alloc.bytes.iter().all(|b| *b == 0) {
                     CStr::from_bytes_with_nul_unchecked(b"__DATA,__thread_bss\0")
                 } else {