arch: riscv: core: Fix type

riscv_cpu_wake_flag and riscv_cpu_sp are variables, not functions, so
they should use the GDATA macro instead of the GTEXT macro. Otherwise,
the linker will warn when LTO is enabled:

ld.bfd: warning: type of symbol `riscv_cpu_wake_flag' changed from 2 to
1 in /tmp/cc39w5oK.ltrans0.ltrans.o
ld.bfd: warning: type of symbol `riscv_cpu_sp' changed from 2 to 1 in
/tmp/cc39w5oK.ltrans0.ltrans.o

Signed-off-by: Tom Hughes <tomhughes@chromium.org>