add missing sub files for mipsel-sf to use softfloat code

the build system has no automatic way to know this code applies to
both big (default) and little endian variants, so explicit .sub files
are needed.

src/fenv/mipsel-sf/fenv.sub

@@ -0,0 +1 @@
+../fenv.c

src/setjmp/mipsel-sf/longjmp.sub

@@ -0,0 +1 @@
+../mips-sf/longjmp.s

src/setjmp/mipsel-sf/setjmp.sub

@@ -0,0 +1 @@
+../mips-sf/setjmp.s