Added u-mode and mstatus.tw to wfe test, cleanup

[silabs-hfegran]

• Added U-mode to wfe test
• Added mstatus.tw to wfe test
• Added test: Illegal instruction when executing wfi with mstatus.tw = in umode
• Added default weak u-mode ecall handler to bsp
• Made default m-mode ecall handler symbol weak
• Updated union members in clic/debug2/wfe-test to use uint32_t as other types have exhibited issues

ci_check ok, no non-sw files modified, so fpv should be unaffected, test itself passes OK (not included in ci_check)