Soon the LLVM upgrade (#34743) will require an updated CMake installation, and
the easiest way to do this was to upgrade the Ubuntu version of the bots to
16.04. This in turn brings in a new MIPS compiler on the linux-cross builder,
which is now from the "official" ubuntu repositories. Unfortunately these
new compilers don't support compiling with the `-msoft-float` flag like we're
currently passing, causing compiles to fail.
This commit removes these flags as it's not clear *why* they're being passed, as
the mipsel targets also don't have it. At least if it's not supported by a
debian default compiler, perhaps it's not too relevant to support?
Currently any compilation to MIPS spits out the warning:
'generic' is not a recognized processor for this target (ignoring processor)
Doesn't make for a great user experience! We don't encounter this in the normal
bootstrap because the cpu/feature set are set by the makefiles. Instead let's
just propagate these to the defaults for the entire target all the time (still
overridable from the command line) and prevent warnings from being emitted by
default.
* Don't pass `-mno-compact-eh`, apparently not all compilers have this?
* Don't pass `+o32`, apparently LLVm doesn't recognize this
* Use `mipsel-linux-gnu` as a prefix instead of `mipsel-unknown-linux-gnu`, this
matches the ubuntu package at least!
