Building for armv6m does not work, building for armv7m/em is fine:
make clean buildbase CROSSINSTALL=1 OS_TARGET=embedded CPU_TARGET=arm SUBARCH=armv6m
/Library/Developer/CommandLineTools/usr/bin/make rtl_all FPC=/Users/ring/devel/fpc/compiler/ppcrossarm
FPCFPMAKE=/Users/ring/devel/fpc/compiler/ppc RELEASE=1 'OPT=' 'FPCMAKEOPT=' /Library/Developer/CommandLineTools/usr/bin/make -C rtl all /Library/Developer/CommandLineTools/usr/bin/make -C embedded all /bin/mkdir -p /Users/ring/devel/fpc/rtl/units/arm-embedded /Users/ring/devel/fpc/compiler/ppcrossarm -Cparmv6m -Ur -Tembedded -Parm -XParm-embedded- -Xr -Ur -Xs -O2 -n -Fi../inc -Fi../arm -FE. -FU/Users/ring/devel/fpc/rtl/units/arm-embedded -darm -dRELEASE -Us -Sg system.pp @system.cfg
/Users/ring/devel/fpc/rtl/units/arm-embedded/system.s: Assembler messages: /Users/ring/devel/fpc/rtl/units/arm-embedded/system.s:57817: Error: lo register required -- `ldr r15,[r0,r1]'
/Users/ring/devel/fpc/rtl/units/arm-embedded/system.s:58293: Error: lo register required -- `ldr r15,[r0,r1]'
/Users/ring/devel/fpc/rtl/units/arm-embedded/system.s:65493: Error: lo register required -- `ldr r15,[r0,r1]'
/Users/ring/devel/fpc/rtl/units/arm-embedded/system.s:72437: Error: lo register required -- `ldr r15,[r0,r1]'
/Users/ring/devel/fpc/rtl/units/arm-embedded/system.s:72797: Error: lo register required -- `ldr r15,[r0,r1]'
/Users/ring/devel/fpc/rtl/units/arm-embedded/system.s:72926: Error: lo register required -- `ldr r15,[r0,r1]'
/Users/ring/devel/fpc/rtl/units/arm-embedded/system.s:73068: Error: lo register required -- `ldr r15,[r0,r1]'
/Users/ring/devel/fpc/rtl/units/arm-embedded/system.s:73214: Error: lo register required -- `ldr r15,[r0,r1]'
system.pp(339) Error: Error while assembling exitcode 1 system.pp(339) Fatal: There were 2 errors compiling module, stopping