feat: add rust.rustflags and per target rustflags options to bootstrap.toml

This makes easy to persistently pass any flag to the compiler when
building rustc.
For example you can use a different linker by putting the following in
`bootstrap.toml`:
```toml
[rust]
rustflags = ["-Clinker=clang", "-Clink-arg=--ld-path=wild"]
```

bootstrap.example.toml

@@ -708,6 +708,12 @@
 # desired in distributions, for example.
 #rust.rpath = true
 
+# Additional flags to pass to `rustc`.
+# Takes precedence over bootstrap's own flags but not over per target rustflags nor env. vars. like RUSTFLAGS.
+# Applies to all stages and targets.
+#
+#rust.rustflags = []
+
 # Indicates whether symbols should be stripped using `-Cstrip=symbols`.
 #rust.strip = false
 
@@ -1013,6 +1019,12 @@
 # and will override the same option under [rust] section. It only works on Unix platforms
 #rpath = rust.rpath (bool)
 
+# Additional flags to pass to `rustc`.
+# Takes precedence over bootstrap's own flags and `rust.rustflags` but not over env. vars. like RUSTFLAGS.
+# Applies to all stages.
+#
+#rustflags = rust.rustflags
+
 # Force static or dynamic linkage of the standard library for this target. If
 # this target is a host for rustc, this will also affect the linkage of the
 # compiler itself. This is useful for building rustc on targets that normally