RISC-V: Allow different dynamic floating point mode to be merged [PR119832]

RISC-V: Allow different dynamic floating point mode to be merged [PR119832]

Although we already try to set the mode needed to FRM_DYN after a function call,
there are still some corner cases where both FRM_DYN and FRM_DYN_CALL may appear
on incoming edges.

Therefore, we use TARGET_MODE_CONFLUENCE to tell GCC that FRM_DYN, FRM_DYN_CALL,
and FRM_DYN_EXIT modes are compatible.

gcc/ChangeLog:

	PR target/119832
	* config/riscv/riscv.cc (riscv_dynamic_frm_mode_p): New.
	(riscv_mode_confluence): New.
	(TARGET_MODE_CONFLUENCE): Define to riscv_mode_confluence.

gcc/testsuite/ChangeLog:

	PR target/119832
	* g++.target/riscv/pr119832.C: New test.