i386: Quote user-defined symbols in assembly in Intel syntax

i386: Quote user-defined symbols in assembly in Intel syntax

With `-masm=intel`, GCC generates registers without % prefixes. If a
user-declared symbol happens to match a register, it will confuse the
assembler. User-defined symbols should be quoted, so they are not to
be mistaken for registers or operators.

Support for quoted symbols were added in Binutils 2.26, originally
for ARM assembly, where registers are also unprefixed:
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=d02603dc201f80cd9d2a1f4b1a16110b1e04222b

This change is required for `@SECREL32` to work in Intel syntax when
targeting Windows, where `@` is allowed as part of a symbol. GNU AS
fails to parse a plain symbol with that suffix:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80881#c79

gcc/ChangeLog:

	PR target/53929
	PR target/80881
	* config/i386/i386-protos.h (ix86_asm_output_labelref): Declare new
	function for quoting user-defined symbols in Intel syntax.
	* config/i386/i386.cc (ix86_asm_output_labelref): Implement it.
	* config/i386/i386.h (ASM_OUTPUT_LABELREF): Use it.
	* config/i386/cygming.h (ASM_OUTPUT_LABELREF): Use it.