share/mk: pass SHARED_CFLAGS to PIC and PIE from assembly

This matches what we do with C sources.  Document SHARED_CFLAGS.

PR:		286975
Reviewed by:	jhb, emaste
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D50466

share/mk/bsd.README

@@ -197,6 +197,9 @@ LIBADD		Additional libraries.  This is for base system libraries
 LIBADD.${.TARGET:T}
 		Libraries dependent on output file name.
 
+SHARED_CFLAGS	Additional compiler flags used when creating shared or
+		position independent object files.
+
 COMPILER_FEATURES
 		A list of features that the compiler supports. Zero or
 		more of:

share/mk/bsd.suffixes-extra.mk

@@ -60,12 +60,13 @@ PIEFLAG?=-fpie
 
 .asm.pico:
 	${CC:N${CCACHE_BIN}} -x assembler-with-cpp ${PICFLAG} -DPIC \
-	    ${CFLAGS} ${ACFLAGS} \
+	    ${SHARED_CFLAGS} ${CFLAGS} ${ACFLAGS} \
 	    -c ${.IMPSRC} -o ${.TARGET}
 	${CTFCONVERT_CMD}
 
 .asm.nossppico:
 	${CC:N${CCACHE_BIN}} -x assembler-with-cpp ${PICFLAG} -DPIC \
+	    ${SHARED_CFLAGS:C/^-fstack-protector.*$//:C/^-fstack-clash-protection.*$//} \
 	    ${CFLAGS:C/^-fstack-protector.*$//:C/^-fstack-clash-protection.*$//} \
 	    ${ACFLAGS} \
 	    -c ${.IMPSRC} -o ${.TARGET}
@@ -73,18 +74,19 @@ PIEFLAG?=-fpie
 
 .asm.pieo:
 	${CC:N${CCACHE_BIN}} -x assembler-with-cpp ${PIEFLAG} -DPIC \
-	    ${CFLAGS} ${ACFLAGS} \
+	    ${SHARED_CFLAGS} ${CFLAGS} ${ACFLAGS} \
 	    -c ${.IMPSRC} -o ${.TARGET}
 	${CTFCONVERT_CMD}
 
 .S.pico:
 	${CC:N${CCACHE_BIN}} ${PICFLAG} -DPIC \
-	    ${CFLAGS} ${ACFLAGS} \
+	    ${SHARED_CFLAGS} ${CFLAGS} ${ACFLAGS} \
 	    -c ${.IMPSRC} -o ${.TARGET}
 	${CTFCONVERT_CMD}
 
 .S.nossppico:
 	${CC:N${CCACHE_BIN}} ${PICFLAG} -DPIC \
+	    ${SHARED_CFLAGS:C/^-fstack-protector.*$//:C/^-fstack-clash-protection.*$//} \
 	    ${CFLAGS:C/^-fstack-protector.*$//:C/^-fstack-clash-protection.*$//} \
 	    ${ACFLAGS} \
 	    -c ${.IMPSRC} -o ${.TARGET}
@@ -92,7 +94,7 @@ PIEFLAG?=-fpie
 
 .S.pieo:
 	${CC:N${CCACHE_BIN}} ${PIEFLAG} -DPIC \
-	    ${CFLAGS} ${ACFLAGS} \
+	    ${SHARED_CFLAGS} ${CFLAGS} ${ACFLAGS} \
 	    -c ${.IMPSRC} -o ${.TARGET}
 	${CTFCONVERT_CMD}