Change the --unpretty flag to -Z unpretty

-Z unpretty no longer requires -Z unstable-options. Also, I mildly
changed the syntax of the flag to match the other -Z flags. All uses of
the flag take the form `unpretty=something` where something can either
`string` or `string=string` (see the help messages of the CLI).

src/test/compile-fail/issue-37665.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags: -Z unstable-options --unpretty=mir
+// compile-flags: -Z unpretty=mir
 // ignore-cloudabi no std::path
 
 use std::path::MAIN_SEPARATOR;

src/test/compile-fail/mir-unpretty.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags: -Z unstable-options --unpretty=mir
+// compile-flags: -Z unpretty=mir
 
 fn main() {
     let x: () = 0; //~ ERROR: mismatched types

src/test/run-make/hir-tree/Makefile

@@ -4,6 +4,5 @@
 # the string constant we would expect to see.
 
 all:
-	$(RUSTC) -o $(TMPDIR)/input.hir -Z unstable-options \
-		--unpretty=hir-tree input.rs
+	$(RUSTC) -o $(TMPDIR)/input.hir -Z unpretty=hir-tree input.rs
 	$(CGREP) '"Hello, Rustaceans!\n"' < $(TMPDIR)/input.hir

src/test/run-make/pretty-print-path-suffix/Makefile

@@ -1,9 +1,9 @@
 -include ../tools.mk
 
 all:
-	$(RUSTC) -o $(TMPDIR)/foo.out -Z unstable-options --unpretty hir=foo input.rs
-	$(RUSTC) -o $(TMPDIR)/nest_foo.out -Z unstable-options --unpretty hir=nest::foo input.rs
-	$(RUSTC) -o $(TMPDIR)/foo_method.out -Z unstable-options --unpretty hir=foo_method input.rs
+	$(RUSTC) -o $(TMPDIR)/foo.out -Z unpretty=hir=foo input.rs
+	$(RUSTC) -o $(TMPDIR)/nest_foo.out -Z unpretty=hir=nest::foo input.rs
+	$(RUSTC) -o $(TMPDIR)/foo_method.out -Z unpretty=hir=foo_method input.rs
 	diff -u $(TMPDIR)/foo.out foo.pp
 	diff -u $(TMPDIR)/nest_foo.out nest_foo.pp
 	diff -u $(TMPDIR)/foo_method.out foo_method.pp