libc: add LIBC_MALLOC option

This will enable alternative mallocs to be included in the tree and
selected by setting LIBC_MALLOC.  As there is only one today (jemalloc)
this option does nothing, but we expect to add other implementations
in the future.  This will also reduce diffs to CheriBSD.

Reviewed by:	emaste
Differential Revision:	https://reviews.freebsd.org/D41660

lib/libc/stdlib/malloc/Makefile.inc

@@ -1,3 +1,3 @@
 SYM_MAPS+=${LIBC_SRCTOP}/stdlib/malloc/Symbol.map
 
-.include "${LIBC_SRCTOP}/stdlib/malloc/jemalloc/Makefile.inc"
+.include "${LIBC_SRCTOP}/stdlib/malloc/${OPT_LIBC_MALLOC}/Makefile.inc"

share/mk/src.opts.mk

@@ -228,6 +228,12 @@ __DEFAULT_DEPENDENT_OPTIONS= \
 	LOADER_VERIEXEC_VECTX/LOADER_VERIEXEC \
 	VERIEXEC/BEARSSL \
 
+__SINGLE_OPTIONS = \
+	LIBC_MALLOC
+
+__LIBC_MALLOC_OPTIONS=	jemalloc
+__LIBC_MALLOC_DEFAULT=	jemalloc
+
 # MK_*_SUPPORT options which default to "yes" unless their corresponding
 # MK_* variable is set to "no".
 #

tools/build/options/LIBC_MALLOC

@@ -0,0 +1,13 @@
+Specify the
+.Xr malloc 3
+implementaiton used by libc.
+The default value is
+.Li jemalloc .
+Valid values are:
+.Bl -tag -width indent
+.It Li jemalloc
+.El
+.Pp
+Other implementations are expected in the future in both
+.Fx
+and downstream consumers.