[MC][X86] Allow SHT_PROGBITS for .eh_frame on x86-64

GNU as emits SHT_PROGBITS .eh_frame by default for .cfi_* directives.
We follow x86-64 psABI and use SHT_X86_64_UNWIND for .eh_frame

Don't error for SHT_PROGBITS .eh_frame on x86-64.
This keeps compatibility with `.section .eh_frame,"a",@progbits` in existing assembly files.

See https://groups.google.com/d/msg/x86-64-abi/7sr4E6THl3g/zUU2UPHOAQAJ
for more discussions.

Reviewed By: joerg

Differential Revision: https://reviews.llvm.org/D76151