[musl] remove SIGSTKFLT on mips?

[enh <enh@google.com>]

(context: https://github.com/landley/toybox/issues/194)

seems like musl defines SIGSTKFLT for mips as signal 7. the kernel
disagrees and says SIGEMT is 7, and that mips doesn't have a
SIGSTKFLT:

arch/mips/include/uapi/asm/signal.h:#define SIGEMT 7

git blame says SIGSTKFLT was introduced in the original commit:

commit 6315004f6102dca44c4ba50654a36967b8b9c2a6
Author: Rich Felker <dalias@aerifal.cx>
Date:   Wed Jul 11 04:22:13 2012 -0400

    initial version of mips (o32) port, based on work by Richard
Pennington (rdp)

    basically, this version of the code was obtained by starting with
    rdp's work from his ellcc source tree, adapting it to musl's build
    system and coding style, auditing the bits headers for discrepencies
    with kernel definitions or glibc/LSB ABI or large file issues, fixing
    up incompatibility with the old binutils from aboriginal linux, and
    adding some new special cases to deal with the oddities of sigaction
    and pipe syscall interfaces on mips.

    at present, minimal test programs work, but some interfaces are broken
    or missing. threaded programs probably will not link.