mk: Don't pass -msoft-float on mips-gnu

Soon the LLVM upgrade (rust-lang#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?

mk/cfg/mips-unknown-linux-gnu.mk

@@ -7,10 +7,10 @@ CFG_LIB_NAME_mips-unknown-linux-gnu=lib$(1).so
 CFG_STATIC_LIB_NAME_mips-unknown-linux-gnu=lib$(1).a
 CFG_LIB_GLOB_mips-unknown-linux-gnu=lib$(1)-*.so
 CFG_LIB_DSYM_GLOB_mips-unknown-linux-gnu=lib$(1)-*.dylib.dSYM
-CFG_JEMALLOC_CFLAGS_mips-unknown-linux-gnu := -mips32r2 -msoft-float -mabi=32 $(CFLAGS)
-CFG_GCCISH_CFLAGS_mips-unknown-linux-gnu := -Wall -g -fPIC -mips32r2 -msoft-float -mabi=32 $(CFLAGS)
+CFG_JEMALLOC_CFLAGS_mips-unknown-linux-gnu := -mips32r2 -mabi=32 $(CFLAGS)
+CFG_GCCISH_CFLAGS_mips-unknown-linux-gnu := -Wall -g -fPIC -mips32r2 -mabi=32 $(CFLAGS)
 CFG_GCCISH_CXXFLAGS_mips-unknown-linux-gnu := -fno-rtti $(CXXFLAGS)
-CFG_GCCISH_LINK_FLAGS_mips-unknown-linux-gnu := -shared -fPIC -g -mips32r2 -msoft-float -mabi=32
+CFG_GCCISH_LINK_FLAGS_mips-unknown-linux-gnu := -shared -fPIC -g -mips32r2 -mabi=32
 CFG_GCCISH_DEF_FLAG_mips-unknown-linux-gnu := -Wl,--export-dynamic,--dynamic-list=
 CFG_LLC_FLAGS_mips-unknown-linux-gnu :=
 CFG_INSTALL_NAME_mips-unknown-linux-gnu =