add some documentation to the unstable book

Author: Jorge Aparicio

src/doc/unstable-book/src/SUMMARY.md

@@ -106,6 +106,7 @@
 - [link_llvm_intrinsics](link-llvm-intrinsics.md)
 - [linkage](linkage.md)
 - [linked_list_extras](linked-list-extras.md)
+- [linker-flavor](linker-flavor.md)
 - [log_syntax](log-syntax.md)
 - [lookup_host](lookup-host.md)
 - [loop_break_value](loop-break-value.md)

src/doc/unstable-book/src/linker-flavor.md

@@ -0,0 +1,61 @@
+# `linker-flavor`
+
+The tracking issue for this feature is: None
+
+------------------------
+
+Every `rustc` target defaults to some linker. For example, Linux targets default
+to gcc. In some cases, you may want to override the default; you can do that
+with the unstable CLI argument: `-Z linker-flavor`.
+
+Here how you would use this flag to link a Rust binary for the
+`thumbv7m-none-eabi` using LLD instead of GCC.
+
+``` text
+$ xargo rustc --target thumbv7m-none-eabi -- \
+    -C linker=ld.lld \
+    -Z linker-flavor=ld \
+    -Z print-link-args | tr ' ' '\n'
+"ld.lld"
+"-L"
+"$SYSROOT/lib/rustlib/thumbv7m-none-eabi/lib"
+"$PWD/target/thumbv7m-none-eabi/debug/deps/app-512e9dbf385f233c.0.o"
+"-o"
+"$PWD/target/thumbv7m-none-eabi/debug/deps/app-512e9dbf385f233c"
+"--gc-sections"
+"-L"
+"$PWD/target/thumbv7m-none-eabi/debug/deps"
+"-L"
+"$PWD/target/debug/deps"
+"-L"
+"$SYSROOT/lib/rustlib/thumbv7m-none-eabi/lib"
+"-Bstatic"
+"$SYSROOT/lib/rustlib/thumbv7m-none-eabi/lib/libcore-e1ccb7dfb1cb9ebb.rlib"
+"-Bdynamic"
+```
+
+Whereas the default is:
+
+```
+$ xargo rustc --target thumbv7m-none-eabi -- \
+    -C link-arg=-nostartfiles \
+    -Z print-link-args | tr ' ' '\n'
+"arm-none-eabi-gcc"
+"-L"
+"$SYSROOT/lib/rustlib/thumbv7m-none-eabi/lib"
+"$PWD/target/thumbv7m-none-eabi/debug/deps/app-961e39416baa38d9.0.o"
+"-o"
+"$PWD/target/thumbv7m-none-eabi/debug/deps/app-961e39416baa38d9"
+"-Wl,--gc-sections"
+"-nodefaultlibs"
+"-L"
+"$PWD/target/thumbv7m-none-eabi/debug/deps"
+"-L"
+"$PWD/target/debug/deps"
+"-L"
+"$SYSROOT/lib/rustlib/thumbv7m-none-eabi/lib"
+"-Wl,-Bstatic"
+"$SYSROOT/lib/rustlib/thumbv7m-none-eabi/lib/libcore-e1ccb7dfb1cb9ebb.rlib"
+"-nostartfiles"
+"-Wl,-Bdynamic"
+```