fix build with toolchain defined _L _U etc constants

[nihui]

do not use underscore prefixed constants defined in linux/include/linux/ctype.h

[codecov-commenter]

Codecov Report

Merging #4957 (3644f56) into master (cbd838f) will not change coverage.
Report is 1 commits behind head on master.
The diff coverage is 99.72%.

@@           Coverage Diff           @@
##           master    #4957   +/-   ##
=======================================
  Coverage   94.90%   94.90%           
=======================================
  Files         776      776           
  Lines      226448   226448           
=======================================
  Hits       214910   214910           
  Misses      11538    11538           

Files Changed	Coverage Δ
src/layer/arm/rnn_arm_asimdhp.cpp	92.52% <96.87%> (ø)
src/layer/arm/gru_arm.cpp	96.58% <100.00%> (ø)
src/layer/arm/gru_arm_asimdhp.cpp	95.03% <100.00%> (ø)
src/layer/arm/interp_bicubic_pack4.h	100.00% <100.00%> (ø)
src/layer/arm/interp_bicubic_pack4_bf16s.h	100.00% <100.00%> (ø)
src/layer/arm/interp_bicubic_pack4_fp16s.h	100.00% <100.00%> (ø)
src/layer/arm/interp_bicubic_pack8_fp16s.h	100.00% <100.00%> (ø)
src/layer/arm/interp_bilinear.h	100.00% <100.00%> (ø)
src/layer/arm/interp_bilinear_bf16s.h	100.00% <100.00%> (ø)
src/layer/arm/interp_bilinear_fp16s.h	100.00% <100.00%> (ø)
... and 29 more