boot: Fall back to objcopy if usable for EFI components

In commit b885643b63 ("boot: Always use ELF Tool Chain elfcopy for EFI
builds") I switched to using elfcopy for EFI_OBJCOPY, as llvm-objcopy
lacks support for translating ELF objects to PE32+.  This broke building
on older releases with WITHOUT_ELFTOOLCHAIN_BOOTSTRAP set, as the host
did not provide elfcopy and it was not built as a bootstrap tool.

Now, if we're not bootstrapping ELF Tool Chain we check the output of
`${OBJCOPY} --version`, and will use that ${OBJCOPY} as long as it is
not llvm-objcopy.

Reviewed by:	imp
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49722

stand/efi/Makefile.inc

@@ -1,3 +1,5 @@
+.include <src.opts.mk>
+
 # Options used when building app-specific efi components
 # See conf/kern.mk for the correct set of these
 CFLAGS+=	-Wformat
@@ -26,7 +28,20 @@ EFI_TARGET=	efi-app-x86_64
 .else
 EFI_TARGET=	binary
 .endif
+
+# GNU or ELF Tool Chain objcopy is usable for building EFI objects.
+# LLVM objcopy is not.  Use elfcopy if we've bootstrapped it (and thus
+# known to be available) or objcopy is LLVM's.
+.if ${MK_ELFTOOLCHAIN_BOOTSTRAP} != "no"
 EFI_OBJCOPY?=	elfcopy
+.else
+_v!=		${OBJCOPY} --version
+.if ${_v:Mllvm-objcopy*} == ""
+EFI_OBJCOPY?=	${OBJCOPY}
+.else
+EFI_OBJCOPY?=	elfcopy
+.endif
+.endif
 
 # Arbitrarily set the PE/COFF header timestamps to 1 Jan 2024 00:00:00
 # for build reproducibility.