RISC-V: T-HEAD: Add support for the XTheadInt ISA extension

The XTheadInt ISA extension provides the following instructions
to accelerate interrupt processing:
* th.ipush
* th.ipop

Ref:
https://github.com/T-head-Semi/thead-extension-spec/releases/download/2.3.0/xthead-2023-11-10-2.3.0.pdf

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (th_int_get_mask): New prototype.
	(th_int_get_save_adjustment): Likewise.
	(th_int_adjust_cfi_prologue): Likewise.
	* config/riscv/riscv.cc (BITSET_P): Moved away from here.
	(TH_INT_INTERRUPT): New macro.
	(riscv_expand_prologue): Add the processing of XTheadInt.
	(riscv_expand_epilogue): Likewise.
	* config/riscv/riscv.h (BITSET_P): Moved to here.
	* config/riscv/riscv.md: New unspec.
	* config/riscv/thead.cc (th_int_get_mask): New function.
	(th_int_get_save_adjustment): Likewise.
	(th_int_adjust_cfi_prologue): Likewise.
	* config/riscv/thead.md (th_int_push): New pattern.
	(th_int_pop): new pattern.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/xtheadint-push-pop.c: New test.