arch/risc-v/cmake: fix linker option case in Toolchain.cmake.

[yanxingyu17]

Summary

Fix lowercase -wl to correct -Wl in arch/risc-v/src/cmake/Toolchain.cmake.

GCC requires uppercase -Wl as the linker option pass-through prefix.
The lowercase -wl,--print-memory-usage is silently ignored, causing
no memory usage report printed during LTO full linking.

Impact

Impact on user: No
Impact on build: Yes, memory usage report now correctly printed for RISC-V LTO full builds.
Impact on hardware: No
Impact on documentation: No
Impact on security: No
Impact on compatibility: No

Testing

Build Host: Ubuntu 22.04, x86_64, GCC 13.2
Target: risc-v, qemu-rv:nsh

Testing logs before change:

(no memory usage output during linking)

Testing logs after change:

Memory region         Used Size  Region Size  %age Used
flash:                  xxx KB       xxx KB     xx.xx%
sram:                   xxx KB       xxx KB     xx.xx%

PR verification Self-Check

• This PR introduces only one functional change.
• I have updated all required description fields above.
• My PR adheres to Contributing Guidelines and Documentation.
• My PR is still work in progress (not ready for review).
• My PR is ready for review and can be safely merged into a codebase.

[fdcavalcanti]

Thank you @yanxingyu17!