testsuite, X86, Darwin: Fix bf16 ABI tests for Mach-O/MacOS ABI.

testsuite, X86, Darwin: Fix bf16 ABI tests for Mach-O/MacOS ABI.

These tests have failed since introduction since they assume that the
assembler output is ELF and that the ABI targeted supports the addressing.

For Darwin, Mach-O and ABI we need to make several changes:
1. Use the __USER_LABEL__PREFIX__
2. Remove the use of ELF-specific constructs (.size, .type etc.)
3. We cannot make direct access to common variables in the ABI, so that we
   must move these to BSS.

These changes are made in darwin-specific asm files.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
gcc/testsuite/ChangeLog:

	* gcc.target/x86_64/abi/bf16/abi-bf16.exp: Use separate asm for Darwin.
	* gcc.target/x86_64/abi/bf16/m256bf16/abi-bf16-ymm.exp: Likewise.
	* gcc.target/x86_64/abi/bf16/m512bf16/abi-bf16-zmm.exp: Likewise.
	* gcc.target/x86_64/abi/bf16/args.h: Make xmm_regs, x87_regs extern.
	* gcc.target/x86_64/abi/bf16/m256bf16/args.h: Likewise.
	* gcc.target/x86_64/abi/bf16/m512bf16/args.h: Likewise.
	* gcc.target/x86_64/abi/bf16/asm-support-darwin.S: New file.
	* gcc.target/x86_64/abi/bf16/m256bf16/asm-support-darwin.S: New file.
	* gcc.target/x86_64/abi/bf16/m512bf16/asm-support-darwin.S: New file.