when building for --target=mips-elf, the mips sim does:
mips.igen:
:include:::micromips.igen
:include:::micromipsdsp.igen

except micromips.igen also does:
:function:::address_word:delayslot_micromips:address_word target, address_word nia, int delayslot_instruction_size
...
  micromips_instruction_decode (SD, CPU, CIA, delayslot_instruction_size);
...

and that function is defined in micromipsrun.c which is not included in the build.  all of which leads to a link failure:
gcc -DHAVE_CONFIG_H    -DPROFILE=1 -DWITH_PROFILE=-1  -DWITH_ALIGNMENT=NONSTRICT_ALIGNMENT -DWITH_TARGET_WORD_BITSIZE=32 -DWITH_TARGET_WORD_MSB=31 -DWITH_DEFAULT_TARGET_BYTE_ORDER=BFD_ENDIAN_BIG  -DWITH_FLOATING_POINT=HARD_FLOATING_POINT -DWITH_TARGET_FLOATING_POINT_BITSIZE=32 -DWITH_HW=0 -DDEFAULT_INLINE=0  -DWITH_RESERVED_BITS=1  -DWITH_SMP=0 -Wall -Wdeclaration-after-statement -Wpointer-arith -Wpointer-sign -Wno-unused -Wunused-value -Wunused-function -Wno-switch -Wno-char-subscripts -Wmissing-prototypes -Wdeclaration-after-statement -Wempty-body -Wmissing-parameter-type -Wold-style-declaration -Wold-style-definition -Wformat-nonliteral    -DMIPS_MACH_DEFAULT=bfd_mach_mips8000   -I. -I../../../../sim/mips -I../common -I../../../../sim/mips/../common -I../../include -I../../../../sim/mips/../..//include -I../../bfd -I../../../../sim/mips/../..//bfd -I../../opcodes -I../../../../sim/mips/../..//opcodes  -g -O2 -static-libstdc++ -static-libgcc  -o run \
  nrun.o libsim.a ../../bfd/libbfd.a ../../opcodes/libopcodes.a  ../../libiberty/libiberty.a -lm -ldl -lnsl  -L../../zlib -lz  
libsim.a(support.o): In function `delayslot_micromips':
/usr/local/src/gnu/gdb/build/build-mips/sim/mips/../../../../sim/mips/micromips.igen:49: undefined reference to `micromips_instruction_decode'
collect2: error: ld returned 1 exit status
Makefile:340: recipe for target 'run' failed
make: *** [run] Error 1

it "worked" in the past because the default inline settings allowed for symbols to be defined in a way (duplicated) that support.o wouldn't get pulled in.  when that's fixed though, support.o gets pulled in, and this link failure is hit.