RISC-V: Support STO_RISCV_VARIANT_CC and DT_RISCV_VARIANT_CC.

This is the original discussion,
https://github.com/riscv/riscv-elf-psabi-doc/pull/190

And here is the glibc part,
https://sourceware.org/pipermail/libc-alpha/2021-August/129931.html

For binutils part, we need to support a new direcitve: .variant_cc.
The function symbol marked by .variant_cc means it need to be resolved
directly without resolver for dynamic linker.  We also add a new dynamic
entry, STO_RISCV_VARIANT_CC, to indicate there are symbols with the
special attribute in the dynamic symbol table of the object.

I heard that llvm already have supported this in their mainline, so
I think it's time to commit this.

bfd/
	* elfnn-riscv.c (riscv_elf_link_hash_table): Added variant_cc
	flag. It is used to check if relocations for variant CC symbols
	may be present.
	(allocate_dynrelocs): If the symbol has STO_RISCV_VARIANT_CC
	flag, then raise the variant_cc flag of riscv_elf_link_hash_table.
	(riscv_elf_size_dynamic_sections): Added dynamic entry for
	variant_cc.
	(riscv_elf_merge_symbol_attribute): New function, used to merge
	non-visibility st_other attributes, including STO_RISCV_VARIANT_CC.
binutils/
	* readelf.c (get_riscv_dynamic_type): New function.
	(get_dynamic_type): Called get_riscv_dynamic_type for riscv targets.
	(get_riscv_symbol_other): New function.
	(get_symbol_other): Called get_riscv_symbol_other for riscv targets.
gas/
	* config/tc-riscv.c (s_variant_cc): Marked symbol that it follows a
	variant CC convention.
	(riscv_elf_copy_symbol_attributes): Same as elf_copy_symbol_attributes,
	but without copying st_other.  If a function symbol has special st_other
	value set via directives, then attaching an IFUNC resolver to that symbol
	should not override the st_other setting.
	(riscv_pseudo_table): Support variant_cc diretive.
	* config/tc-riscv.h (OBJ_COPY_SYMBOL_ATTRIBUTES): Defined.
	* testsuite/gas/riscv/variant_cc-set.d: New testcase.
	* testsuite/gas/riscv/variant_cc-set.s: Likewise.
	* testsuite/gas/riscv/variant_cc.d: Likewise.
	* testsuite/gas/riscv/variant_cc.s: Likewise.
include/
	* elf/riscv.h (DT_RISCV_VARIANT_CC): Defined to (DT_LOPROC + 1).
	(STO_RISCV_VARIANT_CC): Defined to 0x80.
ld/
	* testsuite/ld-riscv-elf/variant_cc-1.s: New testcase.
	* testsuite/ld-riscv-elf/variant_cc-2.s: Likewise.
	* testsuite/ld-riscv-elf/variant_cc-now.d: Likewise.
	* testsuite/ld-riscv-elf/variant_cc-r.d: Likewise.
	* testsuite/ld-riscv-elf/variant_cc-shared.d: Likewise.
	* testsuite/ld-riscv-elf/ld-riscv-elf.exp: Updated.

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