issue about lr/sc and __riscv_atomic

[Xiang W]

There are some conditional compilation related to __riscv_atomic in
lib/sbi/riscv_atomic.c. Atomic operations are implemented through lr/sc when
the __riscv_atomic macro is not defined. But lr/sc is also part of the A
extension. If the A extension is not supported, lr/sc should not be used.

We should remove these conditional compilations, opensbi is strongly dependent
on the A extension.

The following is the compilation log when I turned off the A extension.

make CROSS_COMPILE=riscv64-unknown-elf- PLATFORM_RISCV_ISA=rv64imfdc_zicsr_zifencei
 CC        lib/sbi/riscv_asm.o
 CC        lib/sbi/riscv_atomic.o
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c: Assembler messages:
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:37: Error: unrecognized opcode `amoadd.d.aqrl a5,a1,0(a0)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:37: Error: unrecognized opcode `amoadd.d.aqrl a5,a5,0(a0)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:173: Error: unrecognized opcode `lr.d a3,0(a0)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:175: Error: unrecognized opcode `sc.d.rl a2,a4,0(a0)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:183: Error: unrecognized opcode `lr.d a4,0(a0)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:184: Error: unrecognized opcode `sc.d.rl a3,a5,0(a0)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:194: Error: unrecognized opcode `lr.w a4,0(a0)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:195: Error: unrecognized opcode `sc.w.rl a3,a5,0(a0)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:205: Error: unrecognized opcode `lr.d a4,0(a0)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:206: Error: unrecognized opcode `sc.d.rl a3,a5,0(a0)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:237: Error: unrecognized opcode `amoor.d.aqrl a0,a0,0(a1)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:242: Error: unrecognized opcode `amoand.d.aqrl a0,a0,0(a1)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:237: Error: unrecognized opcode `amoor.d.aqrl a0,a0,0(a1)', extension `a' required
/home/merle/workspaces/opensbi/lib/sbi/riscv_atomic.c:242: Error: unrecognized opcode `amoand.d.aqrl a0,a0,0(a1)', extension `a' required
make: *** [Makefile:504: /home/merle/workspaces/opensbi/build/lib/sbi/riscv_atomic.o] Error 1