libcxx/CMakeLists.txt: Remove unnecessary flags.

[cuiziwei1]

Summary

Impact

build libcxx with cmake.

Testing

ci-test

[acassis]

@cuiziwei1 why is it failing?

Configuration/Tool: smartl-c906/knsh
2024-12-02 11:42:34
------------------------------------------------------------------------------------
  Cleaning...
  Configuring...
  Building NuttX...
common/riscv_exit.c: In function 'up_exit':
Error: common/riscv_exit.c:65:33: error: 'tcb' undeclared (first use in this function); did you mean 'tcb_s'?
   65 |   g_running_tasks[this_cpu()] = tcb;
      |                                 ^~~
      |                                 tcb_s
common/riscv_exit.c:65:33: note: each undeclared identifier is reported only once for each function it appears in
make[1]: *** [Makefile:144: riscv_exit.o] Error 1
make[1]: Target 'libkarch.a' not remade because of errors.
make: *** [tools/LibTargets.mk:59: arch/risc-v/src/libkarch.a] Error 2
make: Target 'all' not remade because of errors.
/github/workspace/sources/nuttx/tools/testbuild.sh: line 385: /github/workspace/sources/nuttx/../nuttx/nuttx.manifest: No such file or directory
  Normalize smartl-c906/knsh

[xiaoxiang781216]

@cuiziwei1 why is it failing?

Configuration/Tool: smartl-c906/knsh
2024-12-02 11:42:34
------------------------------------------------------------------------------------
  Cleaning...
  Configuring...
  Building NuttX...
common/riscv_exit.c: In function 'up_exit':
Error: common/riscv_exit.c:65:33: error: 'tcb' undeclared (first use in this function); did you mean 'tcb_s'?
   65 |   g_running_tasks[this_cpu()] = tcb;
      |                                 ^~~
      |                                 tcb_s
common/riscv_exit.c:65:33: note: each undeclared identifier is reported only once for each function it appears in
make[1]: *** [Makefile:144: riscv_exit.o] Error 1
make[1]: Target 'libkarch.a' not remade because of errors.
make: *** [tools/LibTargets.mk:59: arch/risc-v/src/libkarch.a] Error 2
make: Target 'all' not remade because of errors.
/github/workspace/sources/nuttx/tools/testbuild.sh: line 385: /github/workspace/sources/nuttx/../nuttx/nuttx.manifest: No such file or directory
  Normalize smartl-c906/knsh

Here is the fix: #15014