This code is only relevant when TCG is present in the build. Building
with --disable-tcg --enable-xen on an x86 host we get:
$ ../configure --target-list=x86_64-softmmu,aarch64-softmmu --disable-tcg
--enable-xen
$ make -j$(nproc)
...
libqemu-aarch64-softmmu.fa.p/target_arm_gdbstub.c.o: in function `m_sysreg_ptr':
../target/arm/gdbstub.c:358: undefined reference to `arm_v7m_get_sp_ptr'
../target/arm/gdbstub.c:361: undefined reference to `arm_v7m_get_sp_ptr'
libqemu-aarch64-softmmu.fa.p/target_arm_gdbstub.c.o: in function
`arm_gdb_get_m_systemreg':
../target/arm/gdbstub.c:405: undefined reference to `arm_v7m_mrs_control'