[edk2-platforms][PATCH] Readme.md: Update cross compiling section

[Vin Xue <vinxue@outlook.com>]

1. Update invalid URL link of Linaro GCC toolchains.
2. Add build command example for cross compiling.

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Vin Xue <vinxue@outlook.com>
---
 Readme.md | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/Readme.md b/Readme.md
index ea9492240a..2e30f672cc 100644
--- a/Readme.md
+++ b/Readme.md
@@ -57,10 +57,13 @@ X64                 | x86_64-linux-gnu-
 
 ### GCC
 Linaro provides GCC toolchains for
-[aarch64-linux-gnu](https://releases.linaro.org/components/toolchain/binaries/latest/aarch64-linux-gnu/)
-and [arm-linux-gnueabihf](https://releases.linaro.org/components/toolchain/binaries/latest/arm-linux-gnueabihf/)
-compiled to run on x86_64/i686 Linux and i686 Windows. Some Linux distributions
-provide their own packaged cross-toolchains.
+[aarch64-linux-gnu](https://releases.linaro.org/components/toolchain/binaries/latest-7/aarch64-linux-gnu/)
+and [arm-linux-gnueabihf](https://releases.linaro.org/components/toolchain/binaries/latest-7/arm-linux-gnueabihf/)
+compiled to run on x86_64/i686 Linux and i686 Windows.
+
+Some Linux distributions provide their own packaged cross-toolchains. e.g. Install AArch64 GCC toolchain on Ubuntu (x86_64) with:
+
+`sudo apt install gcc-aarch64-linux-gnu`
 
 ### clang
 Clang does not require separate cross compilers, but it does need a
@@ -160,6 +163,9 @@ variable `{TOOL_CHAIN_TAG}_{TARGET_ARCH}_PREFIX` - in the case above,
 **GCC5_AARCH64_PREFIX**.
 
 So, referring to the cross compiler toolchain table above, we should prepend the `build` command line with `GCC5_AARCH64_PREFIX=aarch64-linux-gnu-`.
+```
+$ GCC5_AARCH64_PREFIX=aarch64-linux-gnu- build -n $NUM_CPUS -a AARCH64 -t GCC5 -p Platform/ARM/JunoPkg/ArmJuno.dsc
+```
 
 ## Using uefi-tools helper scripts
 uefi-tools is a completely unofficial set of helper-scripts developed by Linaro.
-- 
2.27.0.windows.1