Remove PEI_HEADERS define

I noticed a few files double-included libcoff.h, and digging deeper I
found that the PEI_HEADERS define is a sort of external include guard.

This patch adds include guards to the few files in include/coff that
were missing one, and then removes the PEI_HEADERS workaround and the
redundant includes.

I didn't see anything in these files that indicated that
double-inclusion would be useful, so it seems to me that this approach
is ok.

Tested by rebuilding with --enable-targets=all.

2023-08-02  Tom Tromey  <tromey@adacore.com>

	* pei-x86_64.c (PEI_HEADERS): Do not define.
	* pei-loongarch64.c (PEI_HEADERS): Do not define.
	* pei-aarch64.c (PEI_HEADERS): Do not define.
	* pe-x86_64.c (PEI_HEADERS): Do not define.
	* pe-aarch64.c (PEI_HEADERS): Do not define.
	* libpei.h (_LIBPEI_H): Add include guard.
	* coff-x86_64.c (PEI_HEADERS): Do not check.
	* coff-loongarch64.c (PEI_HEADERS): Do not check.
	* coff-aarch64.c (PEI_HEADERS): Do not check.

include/ChangeLog
2023-08-02  Tom Tromey  <tromey@adacore.com>

	* coff/x86_64.h (COFF_X86_64_H): Add include guard.
	* coff/loongarch64.h (COFF_LOONGARCH64_H): Add include guard.
	* coff/aarch64.h (COFF_AARCH64_H): Add include guard.