[scripts] Use do-build instead of doit.

Switch over to using the do-build script, to make it easier to
incorporate the native tools builds.

Change-Id: I6f64642962eace37ada779f40ff2616b34254c49

docs/getting_started.md

@@ -115,8 +115,10 @@ for building Magenta for the ARM64 and x86-64 architectures:
 ```
 cd $SRC
 git clone https://fuchsia.googlesource.com/third_party/gcc_none_toolchains toolchains
-cd toolchains
-./doit -a 'arm aarch64 x86_64' -f -j32
+cd toolchains.
+./do-build --target arm-none
+./do-build --target aarch64-none
+./do-build --target x86_64-none
 ```
 
 ### Configure PATH for toolchains

scripts/upload-toolchain

@@ -32,15 +32,14 @@ if [[ -z "${ARCH}" ]]; then
   exit 99
 fi
 
-# Sanity check the value of ARCH.  `doit` accepts multiple values for arch at a time,
-# so it's easy to accidentally do the same for this script.
+# Sanity check the value of ARCH.
 if [[ "${ARCH}" != "${ARCH// /}" ]]; then
   echo "ARCH appears to contain spaces; provide only one ARCH at a time"
   exit 99
 fi
 
-# Build the toolchain: `doit` will fetch source, apply patches and build gcc and friends.
-./doit -a "${ARCH}" -f -j2 -s
+# Build the toolchain: `do-build` will fetch source, apply patches and build gcc and friends.
+./do-build --target "${ARCH}-none" -j2 --strip
 
 # Figure out where the toolchains got generated.  They end up in a directory with the
 # structure $ARCH-$ABI-$VERSION-$OS-$HOSTARCH.