msun: LIBCSRCDIR is too fragile, use ${SRCTOP}/lib/libc instead

LIBCSRCDIR is defined in bsd.libnames.mk, which is read in later in the
Makefile than the line:

.if exists(${LIBCSRCDIR}/${MACHINE_ARCH})

so we test to see if /${MARCHIN_ARCH} exists which it usually doesn't
(but did for me since I mounted 13.2R SD image there).  Move to defining
our own LIBC_SRCTOP in terms of SRCTOP to treat these uniformily.

Sponsored by:		Netflix
Reviewed by:		sjg
Differential Revision:	https://reviews.freebsd.org/D41661

lib/libc/Makefile

@@ -7,12 +7,14 @@ SHLIBDIR?= /lib
 
 LIBC_SRCTOP?= ${.CURDIR}
 
-# Pick the current architecture directory for libc. In general, this is
-# named MACHINE_CPUARCH, but some ABIs are different enough to require
-# their own libc, so allow a directory named MACHINE_ARCH to override this.
-
-.if exists(${LIBC_SRCTOP}/${MACHINE_ARCH:S/powerpc64le/powerpc64/})
-LIBC_ARCH=${MACHINE_ARCH:S/powerpc64le/powerpc64/}
+# Pick the current architecture directory for libc. In general, this is named
+# MACHINE_CPUARCH, but some ABIs are different enough to require their own libc,
+# so allow a directory named MACHINE_ARCH to override this (though treat
+# powerpc64le and powerpc64 the same).
+# Note: This is copeid to msun/Makefile
+M=${MACHINE_ARCH:S/powerpc64le/powerpc64/}
+.if exists(${LIBC_SRCTOP}/${M})
+LIBC_ARCH=${M}
 .else
 LIBC_ARCH=${MACHINE_CPUARCH}
 .endif

lib/msun/Makefile

@@ -97,17 +97,27 @@ COMMON_SRCS+= s_cospi.c s_cospif.c \
 	s_sinpi.c s_sinpif.c \
 	s_tanpi.c s_tanpif.c
 
+# LIBCSRCDIR is defined lazily, so can't be used for exists() below
+LIBC_SRCTOP=${SRCTOP}/lib/libc
 # Location of fpmath.h and _fpmath.h
-.if exists(${LIBCSRCDIR}/${MACHINE_ARCH})
-LIBC_ARCH=${MACHINE_ARCH}
+
+# Pick the current architecture directory for libc. In general, this is named
+# MACHINE_CPUARCH, but some ABIs are different enough to require their own libc,
+# so allow a directory named MACHINE_ARCH to override this (though treat
+# powerpc64le and powerpc64 the same).
+# Note: This is copied from libc/Makefile
+M=${MACHINE_ARCH:S/powerpc64le/powerpc64/}
+.if exists(${LIBC_SRCTOP}/${M})
+LIBC_ARCH=${M}
 .else
 LIBC_ARCH=${MACHINE_CPUARCH}
 .endif
-CFLAGS+=	-I${.CURDIR}/src -I${LIBCSRCDIR}/include \
-	-I${LIBCSRCDIR}/${LIBC_ARCH}
+
+CFLAGS+=	-I${.CURDIR}/src -I${LIBC_SRCTOP}/include \
+		-I${LIBC_SRCTOP}/${LIBC_ARCH}
 SYM_MAPS+=	${.CURDIR}/Symbol.map
 
-VERSION_DEF=	${LIBCSRCDIR}/Versions.def
+VERSION_DEF=	${LIBC_SRCTOP}/Versions.def
 SYMBOL_MAPS=	${SYM_MAPS}
 
 # C99 long double functions