src.conf: Introduce WITHOUT_MACHDEP knob.

Summary:
This knob can be used to make buildsystem prefer generic C implentations of
various functions, instead of machine-specific assembler ones.

Test Plan: `make buildworld` on amd64

Reviewed by: imp, emaste

Differential Revision: https://reviews.freebsd.org/D36076

MFC after:	3 days

lib/libc/Makefile

@@ -162,7 +162,7 @@ CFLAGS+= -DSYMBOL_VERSIONING
 
 # If there are no machine dependent sources, append all the
 # machine-independent sources:
-.if empty(MDSRCS)
+.if empty(MDSRCS) || ${MK_MACHDEP_OPTIMIZATIONS} == no
 SRCS+=	${MISRCS}
 .else
 # Append machine-dependent sources, then append machine-independent sources

lib/libmd/Makefile

@@ -103,7 +103,7 @@ CFLAGS.skein_block.c+= -DSKEIN_LOOP=995
 .PATH: ${SRCTOP}/sys/crypto/skein ${SRCTOP}/sys/crypto/skein/${MACHINE_ARCH}
 
 USE_ASM_SOURCES?=1
-.if defined(BOOTSTRAPPING)
+.if defined(BOOTSTRAPPING) || ${MK_MACHDEP_OPTIMIZATIONS} == no
 # Don't build ASM sources when bootstrapping to avoid toolchain dependencies
 USE_ASM_SOURCES:=0
 .endif

lib/msun/Makefile

@@ -151,7 +151,7 @@ COMMON_SRCS+=	catrig.c catrigf.c \
 #COMMON_SRCS+=	s_fabs.c s_frexp.c s_isnan.c s_ldexp.c s_modf.c
 
 # Exclude the generic versions of what we provide in the MD area.
-.if defined(ARCH_SRCS)
+.if defined(ARCH_SRCS) && ${MK_MACHDEP_OPTIMIZATIONS} != no
 .for i in ${ARCH_SRCS}
 COMMON_SRCS:=  ${COMMON_SRCS:N${i:R}.c}
 .endfor

share/man/man5/src.conf.5

@@ -1094,6 +1094,8 @@ and related programs.
 Build
 .Xr ls 1
 without support for colors to distinguish file types.
+.It Va WITHOUT_MACHDEP
+Prefer machine-independent non-assembler code across the code base.
 .It Va WITHOUT_MAIL
 Do not build any mail support (MUA or MTA).
 When set, it enforces these options:

share/mk/src.opts.mk

@@ -142,6 +142,7 @@ __DEFAULT_YES_OPTIONS = \
     LOCATE \
     LPR \
     LS_COLORS \
+    MACHDEP_OPTIMIZATIONS \
     MAIL \
     MAILWRAPPER \
     MAKE \

tools/build/options/WITHOUT_MACHDEP_OPTIMIZATIONS

@@ -0,0 +1 @@
+Prefer machine-independent non-assembler code in libc and libm.