Provide default location of sysroot for Baremetal toolchain.

Currently, Baremetal toolchain requires user to pass a sysroot location
using a --sysroot flag. This is not very convenient for the user. It also
creates problem for toolchain vendors who don't have a fixed location to
put the sysroot bits.

Clang does provide 'DEFAULT_SYSROOT' which can be used by the toolchain
builder to provide the default location. But it does not work if toolchain
is targeting multiple targets e.g. arm-none-eabi/riscv64-unknown-elf which
clang is capable of doing.

This patch tries to solve this problem by providing a default location of
the toolchain if user does not explicitly provides --sysroot. The exact
location and name can be different but it should fulfill these conditions:

1. The sysroot path should have a target triple element so that multi-target
toolchain problem (as I described above) could be addressed.

2. The location should not be $TOP/$Triple as this is used by gcc generally
and will be a problem for installing both gcc and clang based toolchain at
the same location.

Reviewed By: jroelofs

Differential Revision: https://reviews.llvm.org/D92677

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