Updates to README.md regarding baremetal.patch

[colematt]

Based on the status of patch D46822 -- which appears to be accepted, closed, and committed to upstream as of July 31, 2018 -- is this step under How can I build upstream LLVM+Clang... still required?

wget https://reviews.llvm.org/D46822?download=true -O baremetal.patch
patch -p0 < baremetal.patch

When I attempted to apply the patch, I got the dreaded "Reversed (or previously applied) patch detected!" error from patch. It seems the patch's contents are now all in the upstream source.

[liang1232018]

I have read the upstream source code git from https://github.com/riscv/riscv-gnu-toolchain .
ALL the patch in baremetal.patch has already been added in the source code.
so , I think it's not required now, And I build the llvm-upstream without that patch successfully