RISC-V: Implement __builtin_thread_pointer

RISC-V: Implement __builtin_thread_pointer

RISC-V has a dedicate register for thread pointer which is specified in psABI
doc, so we could support __builtin_thread_pointer in straightforward way.

Note: clang/llvm was supported __builtin_thread_pointer for RISC-V port
recently.
- https://reviews.llvm.org/rGaabc24acf0d5f8677bd22fe9c108581e07c3e180

gcc/ChangeLog:

	* config/riscv/riscv.md (get_thread_pointer<mode>): New.
	(TP_REGNUM): Ditto.
	* doc/extend.texi (Target Builtins): Add RISC-V built-in section.
	Document __builtin_thread_pointer.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/read-thread-pointer.c: New.