rust: Fix compile error for mipsel_24kc+24kf

rust: Fix compile error for mipsel_24kc+24kf

Currently, rust fails to build for mipsel_24kc+24kf with "opcode not
supported on this processor: mips1 (mips1)" errors when building
libunwind.

Because mipsel_24kc+24kf is hard-float, a certain section of
src/llvm-project/libunwind/src/UnwindRegistersRestore.S is selected to
be compiled; the instructions in this section require MIPS II.

mipsel_24kc+24kf is compiled for MIPS32 Release 2 (MIPS32 is based on
MIPS II), but the C flags used to select this architecture were not
passed to the rust bootstrap (to be passed back to gcc).

This passes the C flags to rust bootstrap to fix this compile error.

This also adds PKG_BUILD_FLAGS:=no-mips16 as attempting to generate
MIPS16 code leads to a different compile error.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>