feat: libcpu/risc-v: unify low-level bringups

This patch consolidates the separated architecture-specific code for
rv64 (virt64 and c906) under a more unified approach. The changes
aim to enhance maintainability and code reuse, reducing duplication
between these two architectures while adding small improvements in
porting compatibility.

Changes:
- Modified build scripts (SConscript) for both virt64 and c906 to
  remove ASID and vector dependencies when not required.
- Updated c906's sbi.c and sbi.h to use standard integer types
  (uint32_t) and include the missing <stdint.h> header.
- Unified inline function declaration for `sbi_call` across both
  c906 and virt64 using `rt_inline`.
- Disabled FPU and vector in c906's startup assembly file, aligning it
  with the virt64 handling.
- Corrected syscall handler type definitions in c906 for consistency.

Signed-off-by: Shell <[email protected]>

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