OS X generates warning about compact debug info for __morestack

[brson]

This is probably not a problem, since the unwinder falls back to dwarf for frames without compact info, but we should figure out a way to suppress it at least. Note that there are other errors in this output.

(stdcore=843d93) rustcrypto $ ls
README.md crypto.rc hash.rs manifest.json pkey.rs symm.rs
(stdcore=843d93) rustcrypto $ rustc crypto.rc
error: linking with gcc failed with code 1
note: gcc arguments: -L/opt/rust/lib/rustc/x86_64-apple-darwin/lib -m64 -o ./libcrypto-aaf71ec8e141741d-0.1.dylib crypto.o -L/opt/rust/lib/rustc/x86_64-apple-darwin/lib -lcore-14bd852465126fe7-0.1 -lcrypto -dynamiclib -Wl,-install_name,@rpath/libcrypto-aaf71ec8e141741d-0.1.dylib -lrustrt -lmorestack -Wl,-rpath,@executable_path/../../lib/rustc/x86_64-apple-darwin/lib -Wl,-rpath,/opt/rust/lib/rustc/x86_64-apple-darwin/lib
note: ld: warning: could not create compact unwind for ___morestack: register saved more than once (might be shrink wrap)
Undefined symbols for architecture x86_64:
  "_EVP_PKEY_get0", referenced from:
      _EVP_PKEY_get0__c_stack_shim in crypto.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

error: aborting due to previous errors
rust: upcall fail 'explicit failure', ../src/comp/driver/session.rs:74
rust: domain main @0x7fa6cb81ea00 root task failed

[brson]

If this is still there we can get rid of it with -Wl,-no_compact_unwind, at the expense of potentially slower unwinding

[brson]

-no_compact_unwind ended up being required