[clang] [MinGW] Guess the right ix86 arch name spelling as sysroot

For x86, most contempory mingw toolchains use i686 as 32 bit
x86 arch target.

As long as the target triple is set to the right form, this works
fine, either as the compiler's default target, or via e.g.
a triple prefix like i686-w64-mingw32-clang.

However, if the unprefixed toolchain targets x86_64, but the user
tries to switch it to target 32 bit by adding the -m32 option, the
computeTargetTriple function in Clang, together with
Triple::get32BitArchVariant, sets the arch to i386. This causes
the right sysroot to not be found.

When targeting an arch where there are potential spelling ambiguities
with respect to the sysroots (i386 and arm), check if the driver can
find a sysroot with the arch name - if not, try a couple other
candidates.

Differential Revision: https://reviews.llvm.org/D111952