I'm having similar issues with sscanf
c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld.exe: c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/thumb/v7e-m+fp/hard\libc.a(libc_a-closer.o): in function `_close_r':
/data/jenkins/workspace/GNU-toolchain/arm-11/src/newlib-cygwin/newlib/libc/reent/closer.c:47: warning: _close is not implemented and will always fail
c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld.exe: c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/thumb/v7e-m+fp/hard\libc.a(libc_a-fstatr.o): in function `_fstat_r':
/data/jenkins/workspace/GNU-toolchain/arm-11/src/newlib-cygwin/newlib/libc/reent/fstatr.c:55: warning: _fstat is not implemented and will always fail
c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld.exe: c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/thumb/v7e-m+fp/hard\libc.a(libc_a-signalr.o): in function `_getpid_r':
/data/jenkins/workspace/GNU-toolchain/arm-11/src/newlib-cygwin/newlib/libc/reent/signalr.c:83: warning: _getpid is not implemented and will always fail
c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld.exe: c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/thumb/v7e-m+fp/hard\libc.a(libc_a-isattyr.o): in function `_isatty_r':
/data/jenkins/workspace/GNU-toolchain/arm-11/src/newlib-cygwin/newlib/libc/reent/isattyr.c:52: warning: _isatty is not implemented and will always fail
c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld.exe: c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/thumb/v7e-m+fp/hard\libc.a(libc_a-signalr.o): in function `_kill_r':
/data/jenkins/workspace/GNU-toolchain/arm-11/src/newlib-cygwin/newlib/libc/reent/signalr.c:53: warning: _kill is not implemented and will always fail
c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld.exe: c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/thumb/v7e-m+fp/hard\libc.a(libc_a-lseekr.o): in function `_lseek_r':
/data/jenkins/workspace/GNU-toolchain/arm-11/src/newlib-cygwin/newlib/libc/reent/lseekr.c:49: warning: _lseek is not implemented and will always fail
c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld.exe: c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/thumb/v7e-m+fp/hard\libc.a(libc_a-readr.o): in function `_read_r':
/data/jenkins/workspace/GNU-toolchain/arm-11/src/newlib-cygwin/newlib/libc/reent/readr.c:49: warning: _read is not implemented and will always fail
c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/../../../../arm-none-eabi/bin/ld.exe: c:/progra~2/armgnu~1/1142da~1.3re/bin/../lib/gcc/arm-none-eabi/11.3.1/thumb/v7e-m+fp/hard\libc.a(libc_a-writer.o): in function `_write_r':
/data/jenkins/workspace/GNU-toolchain/arm-11/src/newlib-cygwin/newlib/libc/reent/writer.c:49: warning: _write is not implemented and will always fail
I'm not comfortable with workarounds that involve altering SDK make/etc. files directly, as they'd just disappear (and then break the build) when updating the SDK 
At this point, I'll just roll my own alternative. It would help, though, if there were a definitive (white?)list of what is actually supported from the C libraries. 
Or, is there a definitive list of things from the C libraries that aren't supported?
Edit: I had to go and tweak common.mk anyway, as I kept running into more and more 'things that aren't supported' (atof, strtof, etc. etc.). I made a copy of it with the changes and set it aside to remind me to tweak common.mk again when updating the SDK.
Edit edit: no, this still isn't working. Looks like I get to write my own 'atof' from scratch... fun!
(note to self: no matter how cute the console may look, remember that it's an embedded system - you don't get all the bells and whistles and should expect this sort of thing )