Makefile: Use pipes rather than temporary files for intermediate steps

GCC supports the use of pipes for intermediate compilation steps (e.g.
passing the generated assembly code to the assembler) as a replacement for
temporary files. This bypasses VFS and other layers which can introduce
substantial amounts of overhead and instead redirects data directly
between processes.

The final product and generated code are unaffected. Memory usage while
compiling is slightly higher.

Tests showed a substantial reduction in build time when using GCC to
compile an x86 4.19 kernel:
Using temporary files in tmpfs: 2m41s
Using pipes:                    2m36s

Similar benefits were observed with an Android arm64 4.9 kernel:
Using tmpfs: 5m34s
Using pipes: 4m33s

Enable the feature when possible (i.e. when the compiler supports it) to
speed up builds at effectively no cost for many setups, particularly
those with weaker CPUs.

Test: kernel compiles and boots
Signed-off-by: Danny Lin <[email protected]>
Signed-off-by: Alexander Winkowski <[email protected]>

Makefile

@@ -439,7 +439,7 @@ endif
 
 KBUILD_USERHOSTCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
 			 -O2 -fomit-frame-pointer -std=gnu11 \
-			 -Wdeclaration-after-statement
+			 -Wdeclaration-after-statement -pipe
 KBUILD_USERCFLAGS  := $(KBUILD_USERHOSTCFLAGS) $(USERCFLAGS)
 KBUILD_USERLDFLAGS := $(USERLDFLAGS)
 
@@ -523,7 +523,7 @@ KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
 		   -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE \
 		   -Werror=implicit-function-declaration -Werror=implicit-int \
 		   -Werror=return-type -Wno-format-security \
-		   -std=gnu11
+		   -std=gnu11 -pipe
 KBUILD_CPPFLAGS := -D__KERNEL__
 KBUILD_AFLAGS_KERNEL :=
 KBUILD_CFLAGS_KERNEL :=