Spurious SIGILL (signal 4) when linking with cc on macOS during doctest

[kennytm]

Symptom: The Linker crashes with signal 4, e.g.

error: linking with `cc` failed: signal: 4

Previous instances:

Date	URL	Failing test
2018-08-31T21:30:20Z	#53533 (comment)	alloc.rs - alloc::alloc (line 62)
2018-09-29T19:38:06Z	#54601 (comment)	hex.rs - hex::[u8]::to_hex (line 32)
2018-10-17T16:28:07Z	#55134 (comment)	../stdsimd/coresimd/macros.rs - coresimd::x86::__m128 (line 27)
2018-10-17T23:16:07Z	#54349 (comment)	src/test/rustdoc/test_option_check/bar.rs - bar::foooo (line 16)
2018-11-23T08:50:08Z	#55767 (comment)	alloc.rs - alloc::System (line 106)
2018-11-30T11:50:06Z	#56214 (comment)	/Users/travis/build/rust-lang/rust/src/doc/unstable-book/src/language-features/generators.md - generators::_::Generators_as_state_machines (line 166)
2018-12-02T05:06:07Z	#55010 (comment)	src/test/rustdoc/test_option_check/bar.rs - bar::foooo (line 16)
2018-12-09T16:14:08Z	#56463 (comment)	../stdsimd/coresimd/macros.rs - coresimd::x86::__m128 (line 292)
2018-12-11T03:22:07Z	#56092 (comment)	alloc.rs - alloc::alloc (line 66)
2019-01-20T08:26:07Z	#57647 (comment)	src/test/rustdoc/test_option_check/bar.rs - bar::foooo (line 6)

[wesleywiser]

Closing since this hasn't happened in a long time.