Fix in-tree GCC builds after r304681.

There were a few issues.
- In-tree GCC won't have X_COMPILER_TYPE defined but will have
  WANT_COMPILER_TYPE==gcc set from the SYSTEM_COMPILER logic that can
  be used.  Make the clang check specific to clang as well to ensure
  -target doesn't leak into a GCC build.
- When using a cross-compiler GCC (with a default sysroot or arch) and also
  passing --sysroot, it basically forgets all internal paths for
  libraries.  We've already worked around this quite a bit for
  the external toolchains.  Now for the in-tree bootstrap cross-compiler
  GCC, also pass in the needed -B${WORLDTMP}/usr/lib to find the crt
  object files, but also -isystem and -L to fix the paths.  This creates
  quite a spammy build log, but it is clear and still achieves the goals
  and stays consistent between internal and external build flags.
  Reducing the spam by using the '=' prefix feature will help and be
  done later.

MFC after:	3 days
X-MFC-With:	r304681
Reported by:	bz
Pointyhat to:	bdrewery
Sponsored by:	EMC / Isilon Storage Division

Makefile.inc1

@@ -572,18 +572,23 @@ TARGET_ABI=	gnueabihf
 TARGET_ABI=	gnueabi
 .endif
 .endif
-.if defined(X_COMPILER_TYPE) && ${X_COMPILER_TYPE} == gcc
+.if ${WANT_COMPILER_TYPE} == gcc || \
+    (defined(X_COMPILER_TYPE) && ${X_COMPILER_TYPE} == gcc)
 # GCC requires -isystem and -L when using a cross-compiler.  --sysroot
 # won't set header path and -L is used to ensure the base library path
 # is added before the port PREFIX library path.
 XCFLAGS+=	-isystem ${WORLDTMP}/usr/include -L${WORLDTMP}/usr/lib
+# GCC requires -B to find /usr/lib/crti.o when using a cross-compiler
+# combined with --sysroot.
+XCFLAGS+=	-B${WORLDTMP}/usr/lib
 # Force using libc++ for external GCC.
 # XXX: This should be checking MK_GNUCXX == no
 .if ${X_COMPILER_VERSION} >= 40800
 XCXXFLAGS+=	-isystem ${WORLDTMP}/usr/include/c++/v1 -std=c++11 \
 		-nostdinc++ -L${WORLDTMP}/../lib/libc++
 .endif
-.else
+.elif ${WANT_COMPILER_TYPE} == clang || \
+    (defined(X_COMPILER_TYPE) && ${X_COMPILER_TYPE} == clang)
 TARGET_ABI?=	unknown
 TARGET_TRIPLE?=	${TARGET_ARCH:C/amd64/x86_64/}-${TARGET_ABI}-freebsd12.0
 XCFLAGS+=	-target ${TARGET_TRIPLE}

Makefile.libcompat

@@ -73,7 +73,8 @@ LIBCOMPATCFLAGS+=	${LIBCOMPATCPUFLAGS} \
 # Clang/GCC.
 LIBCOMPATCFLAGS+=	-B${LIBCOMPATTMP}/usr/lib${libcompat}
 
-.if defined(X_COMPILER_TYPE) && ${X_COMPILER_TYPE} == gcc
+.if ${WANT_COMPILER_TYPE} == gcc || \
+    (defined(X_COMPILER_TYPE) && ${X_COMPILER_TYPE} == gcc)
 # GCC requires -isystem when using a cross-compiler and --sysroot.  Note that
 # Makefile.inc1 only applies this with an external compiler but libcompat
 # always does since even in-tree GCC 4.2 needs this to override the built-in