lang/rust: Disable "omit frame pointers"

Before the patch, the default behavior of the compiler was to omit frame
pointers. However, even though it looks fine for the code produced
by the compiler, the compiler itself was broken. When there was a
compilation error, rustc panics to report that error. While handling
that panic, it tries to unwind the stack but fails to do so, which
results in an abort(3).

Here is an example:

    $ rustc hello.rs
    error: invalid reference to argument `0` (no arguments given)
     --> hello.rs:2:5
      |
    2 |     println!("Hello {}")
      |     ^^^^^^^^^^^^^^^^^^^^
      |
      = note: this error originates in a macro outside of the current crate

    error: aborting due to previous error

    fatal runtime error: failed to initiate panic, error 5
    Abort trap (core dumped)

The problem also exists on other systems such as OpenBSD and
i686-unknown-linux-musl. "omit frame pointers" was already disabled for
i686-unknown-linux-musl:
88a28ff602/src/librustc_back/target/i686_unknown_linux_musl.rs (L22)

The patch does the same for *-unknown-freebsd.

See the upstream bug for more informations:
https://github.com/rust-lang/rust/issues/43575

lang/rust/Makefile

@@ -3,7 +3,7 @@
 
 PORTNAME=	rust
 PORTVERSION?=	1.22.1
-PORTREVISION?=	1
+PORTREVISION?=	2
 CATEGORIES=	lang
 MASTER_SITES=	http://static.rust-lang.org/dist/:src \
 		LOCAL/dumbbell/rust:rust_bootstrap \

lang/rust/files/patch-src_librustc__back_target_freebsd__base.rs

@@ -1,12 +1,13 @@
 Return struct like OpenBSD, see src/librustc_trans/cabi_x86.rs
 
---- src/librustc_back/target/freebsd_base.rs.orig	2017-07-17 22:10:50 UTC
+--- src/librustc_back/target/freebsd_base.rs.orig	2017-11-22 21:33:00 UTC
 +++ src/librustc_back/target/freebsd_base.rs
-@@ -31,6 +31,7 @@ pub fn opts() -> TargetOptions {
+@@ -31,6 +31,8 @@ pub fn opts() -> TargetOptions {
          target_family: Some("unix".to_string()),
          linker_is_gnu: true,
          has_rpath: true,
 +        is_like_openbsd: true,
++        eliminate_frame_pointer: false,
          pre_link_args: args,
          position_independent_executables: true,
-         exe_allocation_crate: super::maybe_jemalloc(),
+         relro_level: RelroLevel::Full,